WO2008059276A2 - Droplet volume control - Google Patents

Droplet volume control Download PDF

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Publication number
WO2008059276A2
WO2008059276A2 PCT/GB2007/004396 GB2007004396W WO2008059276A2 WO 2008059276 A2 WO2008059276 A2 WO 2008059276A2 GB 2007004396 W GB2007004396 W GB 2007004396W WO 2008059276 A2 WO2008059276 A2 WO 2008059276A2
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WO
WIPO (PCT)
Prior art keywords
nozzle
nozzles
driving signal
volume
ink
Prior art date
Application number
PCT/GB2007/004396
Other languages
French (fr)
Other versions
WO2008059276A3 (en
Inventor
Mark Bale
Laura Webb
Original Assignee
Cambridge Display Technology Limited
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Filing date
Publication date
Application filed by Cambridge Display Technology Limited filed Critical Cambridge Display Technology Limited
Publication of WO2008059276A2 publication Critical patent/WO2008059276A2/en
Publication of WO2008059276A3 publication Critical patent/WO2008059276A3/en

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Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
    • B41J—TYPEWRITERS; SELECTIVE PRINTING MECHANISMS, i.e. MECHANISMS PRINTING OTHERWISE THAN FROM A FORME; CORRECTION OF TYPOGRAPHICAL ERRORS
    • B41J2/00—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed
    • B41J2/005—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed characterised by bringing liquid or particles selectively into contact with a printing material
    • B41J2/01—Ink jet
    • B41J2/015—Ink jet characterised by the jet generation process
    • B41J2/04—Ink jet characterised by the jet generation process generating single droplets or particles on demand
    • B41J2/045—Ink jet characterised by the jet generation process generating single droplets or particles on demand by pressure, e.g. electromechanical transducers
    • B41J2/04501—Control methods or devices therefor, e.g. driver circuits, control circuits
    • B41J2/04581—Control methods or devices therefor, e.g. driver circuits, control circuits controlling heads based on piezoelectric elements
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
    • B41J—TYPEWRITERS; SELECTIVE PRINTING MECHANISMS, i.e. MECHANISMS PRINTING OTHERWISE THAN FROM A FORME; CORRECTION OF TYPOGRAPHICAL ERRORS
    • B41J2/00—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed
    • B41J2/005—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed characterised by bringing liquid or particles selectively into contact with a printing material
    • B41J2/01—Ink jet
    • B41J2/015—Ink jet characterised by the jet generation process
    • B41J2/04—Ink jet characterised by the jet generation process generating single droplets or particles on demand
    • B41J2/045—Ink jet characterised by the jet generation process generating single droplets or particles on demand by pressure, e.g. electromechanical transducers
    • B41J2/04501—Control methods or devices therefor, e.g. driver circuits, control circuits
    • B41J2/04506—Control methods or devices therefor, e.g. driver circuits, control circuits aiming at correcting manufacturing tolerances
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
    • B41J—TYPEWRITERS; SELECTIVE PRINTING MECHANISMS, i.e. MECHANISMS PRINTING OTHERWISE THAN FROM A FORME; CORRECTION OF TYPOGRAPHICAL ERRORS
    • B41J2/00—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed
    • B41J2/005—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed characterised by bringing liquid or particles selectively into contact with a printing material
    • B41J2/01—Ink jet
    • B41J2/015—Ink jet characterised by the jet generation process
    • B41J2/04—Ink jet characterised by the jet generation process generating single droplets or particles on demand
    • B41J2/045—Ink jet characterised by the jet generation process generating single droplets or particles on demand by pressure, e.g. electromechanical transducers
    • B41J2/04501—Control methods or devices therefor, e.g. driver circuits, control circuits
    • B41J2/0456—Control methods or devices therefor, e.g. driver circuits, control circuits detecting drop size, volume or weight
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
    • B41J—TYPEWRITERS; SELECTIVE PRINTING MECHANISMS, i.e. MECHANISMS PRINTING OTHERWISE THAN FROM A FORME; CORRECTION OF TYPOGRAPHICAL ERRORS
    • B41J2/00—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed
    • B41J2/005—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed characterised by bringing liquid or particles selectively into contact with a printing material
    • B41J2/01—Ink jet
    • B41J2/015—Ink jet characterised by the jet generation process
    • B41J2/04—Ink jet characterised by the jet generation process generating single droplets or particles on demand
    • B41J2/045—Ink jet characterised by the jet generation process generating single droplets or particles on demand by pressure, e.g. electromechanical transducers
    • B41J2/04501—Control methods or devices therefor, e.g. driver circuits, control circuits
    • B41J2/0458—Control methods or devices therefor, e.g. driver circuits, control circuits controlling heads based on heating elements forming bubbles
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B5/00—Optical elements other than lenses
    • G02B5/20—Filters
    • G02B5/201—Filters in the form of arrays
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K71/00—Manufacture or treatment specially adapted for the organic devices covered by this subclass
    • H10K71/10—Deposition of organic active material
    • H10K71/12—Deposition of organic active material using liquid deposition, e.g. spin coating
    • H10K71/13—Deposition of organic active material using liquid deposition, e.g. spin coating using printing techniques, e.g. ink-jet printing or screen printing
    • H10K71/135—Deposition of organic active material using liquid deposition, e.g. spin coating using printing techniques, e.g. ink-jet printing or screen printing using ink-jet printing

Definitions

  • This invention generally relates to the deposition of material for electronic devices, particularly molecular electronic devices such as organic light emitting diodes, by an ink jet-type process.
  • the invention is particularly concerned with droplet volume measurement and calibration.
  • Organic light emitting diodes comprise a particularly advantageous form of electro-optic display. They are bright, stylish, fast-switching, provide a wide viewing angle and are easy and cheap to fabricate on a variety of substrates.
  • Organic LEDs may be fabricated using either polymers or small molecules in a range of colours (or in multi-coloured displays), depending upon the materials used. Examples of polymer- based organic LEDs are described in WO 90/13148, WO 95/06400 and WO 99/48160; examples of so-called small molecule based devices are described in US 4,539,507.
  • a basic structure 100 of a typical organic LED is shown in Figure Ia.
  • the OLED 100 comprises a substrate 102, typically 0.7 mm or 1.1 mm glass but optionally clear plastic or some other substantially transparent material.
  • An anode layer 104 is deposited on the substrate, typically comprising around 40 to 150 nm thickness of ITO (indium tin oxide), over part of which is provided a metal contact layer.
  • ITO indium tin oxide
  • the contact layer comprises around 500nm of aluminium, or a layer of aluminium sandwiched between layers of chrome, and this is sometimes referred to as anode metal.
  • Glass substrates coated with ITO and contact metal are widely available. The contact metal over the ITO helps provide reduced resistance pathways where the anode connections do not need to be transparent, in particular for external contacts to the device.
  • the contact metal is removed from the ITO where it is not wanted, in particular where it would otherwise obscure the display, by a standard process of photolithography followed by etching.
  • a substantially transparent hole injection layer 106 is deposited over the anode layer, followed by an electroluminescent layer 108, and a cathode 110.
  • the electroluminescent layer 108 may comprise, for example, a PPV (poly(p- phenylenevinylene)) and the hole injection layer 106, which helps match the hole energy levels of the anode layer 104 and electroluminescent layer 108, may comprise a conductive transparent polymer, for example PEDOT:PSS (polystyrene-sulphonate- doped polyethylene-dioxythiophene) from H.C. Starck of Germany.
  • PEDOT:PSS polystyrene-sulphonate- doped polyethylene-dioxythiophene
  • the hole injection layer 106 may comprise around 200 nm of PEDOT.
  • the light emitting polymer layer 108 is typically around 70 nm in thickness.
  • These organic layers may be deposited by spin coating (afterwards removing material from unwanted areas by plasma etching or laser ablation) or by inkjet printing.
  • banks 112 may be formed on the substrate, for example using photoresist, to define wells into which the organic layers maybe deposited.
  • Such wells define light emitting areas or pixels of the display.
  • Cathode layer 110 typically comprises a low work function metal such as calcium or barium (for example deposited by physical vapour deposition) covered with a thicker, capping layer of aluminium.
  • a low work function metal such as calcium or barium (for example deposited by physical vapour deposition) covered with a thicker, capping layer of aluminium.
  • an additional layer may be provided immediately adjacent the electroluminescent layer, such as a layer of lithium fluoride, for improved electron energy level matching.
  • Mutual electrical isolation of cathode lines may be achieved or enhanced through the use of cathode separators (not shown in Figure 1).
  • the same basic structure may also be employed for small molecule devices.
  • the displays typically are fabricated on a single substrate and at the end of the fabrication process the substrate is scribed, and the displays separated before an encapsulating can is attached to each to inhibit oxidation and moisture ingress.
  • the displays can be encapsulated prior to scribing and separating.
  • top emitters Devices which emit through the cathode (“top emitters”) may also be constructed, for example, by keeping the thickness of cathode layer 110 less than around 50-100 nm so that the cathode is substantially transparent and/or using a transparent cathode material such as ITO.
  • FIG. Ib shows a simplified cross-section through a passive matrix OLED display device 150, in which like elements to those of Figure 1 are indicated by like reference numerals.
  • the hole injection layer 106 and the electroluminescent layer 108 are subdivided into a plurality of pixels 152 at the intersection of mutually perpendicular anode and cathode lines defined in the anode metal 104 and cathode layer 110 respectively.
  • conductive lines 154 defined in the cathode layer 110 run into the page and a cross-section through one of a plurality of anode lines 158 running at right angles to the cathode lines is shown.
  • An electroluminescent pixel 152 at the intersection of a cathode and anode line may be addressed by applying a voltage between the relevant lines.
  • the anode metal layer 104 provides external contacts to the display 150 and may be used for both anode and cathode connections to the OLEDs (by running the cathode layer pattern over anode metal lead-outs).
  • the above mentioned OLED materials, and in particular the light emitting polymer material and the cathode, are susceptible to oxidation and to moisture.
  • the device is therefore encapsulated in a metal or glass can 111, attached by UV-curable epoxy glue 113 onto anode metal layer 104.
  • the anode metal contacts are thinned where they pass under the lip of the metal can 111 to facilitate exposure of glue 113 to UV light for curing.
  • OLEDs organic light emitting diodes
  • T.R. Hebner, CC. Wu, D. Marcy, M.H. Lu and J.C. Sturm "Ink-jet Printing of doped Polymers for Organic Light Emitting Devices", Applied Physics Letters, Vol. 72, No. 5, pp.519- 521, 1998; Y. Yang, “Review of Recent Progress on Polymer Electroluminescent Devices,” SPIE Photonics West: Optoelectronics '98, Conf. 3279, San Jose, Jan., 1998; EP O 880 303; and "Ink- Jet Printing of Polymer Light-Emitting Devices", Paul C.
  • InkJet techniques can be used to deposit materials for both small molecule and polymer LEDs, although these applications present their own particular problems, which are different to the problems encountered in conventional ink jet printing of images on paper or plastic, as will be explained more fully below.
  • ink in the following disclosure is taken to mean a dissolved molecular electronic material, which can include semi conductor material, Light Emitting Polymers (LEP) or small molecules.
  • Figures 2a and 2b which are taken from EP 1,219,980, show ink jet printing apparatus which may be employed for this type of application.
  • Figure 2a shows an ink jet printer 200 comprising a base 209 supporting first and second linear positioners 206, 208 for moving a substrate 212 and ink jet print head 222 relative to one another along two orthogonal axis Y and X.
  • Positioner 206 comprises a pair of rails 254 mounting a slider 256 provided with a turntable 251 supporting a table or bed 249 on which the substrate 212 is supported.
  • the substrate 212 is aligned on table or bed 249 by means of stops 250 against which two edges of the substrate abut.
  • Turntable 251 allows the table and substrate 249, 212 to be rotated relative to the print head 222.
  • Positioner 208 comprises a pair of rails 252 mounting a slider 253 which carries rotary positioners 244, 246, 247 which allow a print head unit 226 carrying the print head to be rotated independently about three orthogonal axes.
  • a further linear positioner 248 is also mounted on slider 253 to allow the print head unit and print head to be translated in the Z-direction, that is towards and away from substrate 212.
  • Ink jet printer system 200 is controlled by a computer terminal 202 via an umbilical 204.
  • Terminal 202 may comprise a general purpose computer with interface hardware for interfacing to the above-described linear and rotary positioners, running operating system, user interface and other ink jet printer drive and control software, in a conventional manner.
  • terminal 202 typically includes a data input device such as a network interface of floppy disk drive for receiving data defining a pattern to be printed, and printer control software to control the printer hardware to print a pattern in accordance with stored or input data.
  • Other conventional functions such as test functions, head cleaning functions and the like are generally also provided by software running on terminal 202.
  • Figure 2b shows print head 222 in more detail.
  • the print head has a plurality of nozzles 227, typically orifices in a nozzle plate for ejecting droplets of fluid from the print head onto the substrate.
  • a fluid supply for printing may either be provided by a reservoir within the print head or print head unit or fluid may be supplied from an external source.
  • the print head 222 has a single row 228 of nozzles 227, but in other examples of print heads more than one row of nozzles may be provided with nozzles offset in one or two dimensions.
  • the diameter of the orifices of nozzles 227 is typically between lO ⁇ m and lOO ⁇ m, and drop sizes are similar.
  • the space or pitch between adjacent nozzle orifices is typically between 50 ⁇ m and 100/rni.
  • Figure 3 a shows a conventional printing strategy in which print head 222 prints successive swathes 302, 304 in the Y-direction, stepping in the X-direction between each swathe.
  • the technique illustrated in Figure 3b may be employed to produce a finer dot pitch.
  • the print head is positioned at an angle ⁇ to the X-direction to reduce the dot pitch by a factor of cos ⁇ .
  • Figure 3c shows two examples 306 and 308 of the distribution of drop volume ejected from nozzles 227 across the width of print head 222.
  • a volatile solvent such as toluene or xylene is employed with 1-2% dissolved solvent material. This results in a relatively thin film in comparison with the initial "ink" volume.
  • the drying time is dependent upon the solvent mix and the atmosphere above the substrate, but typically varies between a few seconds and some minutes. It is strongly preferable all the drops comprising material which are eventually to make up a pixel are deposited before drying begins.
  • Solvents which may be used include alkylated benzenes, in particular toluene or xylene. Other solvents for inkjet printing are described in WO 00/59267, WO 01/16251 and WO 02/18513.
  • the pattern of material to be deposited is made up of pixels formed by depositing the electroluminescent material into a well (as described, for example, in EP 0 880 303) on a substrate.
  • the wells are usually formed by photolithography of a photoresist as described in EP 0 862 156 to which reference may be made.
  • OLEDs and other molecular electronic devices such as polymer FETs (Field Effect Transistors) these pixels and wells generally have regular shapes and a regular pattern, but in other cases the pixels can have irregular shapes.
  • the substrate typically comprises a substantially non-absorbent material such as, for OLED displays, glass, clear plastics such as polyethylene or PET or other materials such as polyvinylidene fluoride or polyimide.
  • the pixels are typically around 50 ⁇ m wide and 40- 50 ⁇ m long in a colour display or approximately three times this length in a monochrome display.
  • the pixel spacing is typically 10-20 ⁇ m.
  • the print head is typically around lcm wide and a few centimetres long.
  • InkJet printing processes may also be used in the creation of thin film transistors (TFT).
  • TFT thin film transistors
  • the TFT structure comprises a substrate 400 on which is deposited a gate electrode 402 followed by a dielectric layer 404 (for example, BCB (Benzocyclobutene); also inorganic materials such as SjO x or SjN x ) and source and drain electrodes 406, 408.
  • a layer of organic thin film transistor material 410 generally an organic semiconductor such as a polythiophene derivative is then deposited over the source and drain and dielectric layer.
  • InkJet printing processes are useable in at least the deposition of the organic semiconductor and dielectric materials.
  • One known strategy for more accurately controlling the volume of material deposited is to cover a pixel or fill a well using a plurality of sequentially deposited drops rather than a single drop, and this strategy is described in EP 1,219,980, in which the print head makes multiple passes in the Y-direction (referring to Figure 3a), depositing one drop onto a pixel on each pass.
  • this has the disadvantage that there is a relatively long period between successive drops landing on a single pixel, which can result in undesirable artefacts.
  • a zig-zag scanning strategy is adopted for the X-direction the intervals between successive drops landing is non-uniform, depending upon the position of a pixel in the X-direction.
  • EP 1,219,980 With the technique described in EP 1,219,980 a slow drying solvent must be employed to prevent drying between successive swathes, but a greater flexibility in solvent choice is preferable and for some applications relatively quick drying solvents such as toluene and xylene, for example with drop drying times of the order of one second, are useful.
  • the technique of EP 1,219,980 is directed towards averaging out drop landing errors (thus reducing "banding") as much as averaging out drop volume variations.
  • One technique for drop volume control is to calibrate one, or preferably a plurality of nozzles of a printhead by measuring the volume of an ejected drop whilst in flight for a range of printhead drive signals. Data collected in this way may then be used to determine or adjust a printhead drive signal in order to obtain a desired drop volume.
  • a calibration procedure may be performed as part of a commissioning process for ink jet or droplet-based deposition apparatus, or a calibration procedure may be performed by the apparatus at switch on.
  • FIG. 5 shows (not to scale) equipment 600 which may be employed to determine the volume of a droplet of dissolved material ejected from a droplet deposition head 602 of droplet deposition apparatus, such as ink jet print head of an ink jet-type printer, hi Figure 5 a droplet 606 of dissolved material has been ejected from a nozzle 604 of print head 602 and is in flight towards a substrate 608. Whilst droplet 606 is in flight it is illuminated from an illumination source 610, for example comprising a strobed LED (Light Emitting Diode) 612 and a lens 614. Illumination is directed at droplet 606 by means of a beam splitter 616 and droplet 606 is viewed through beam splitter 616 by a digital camera 618 capable of capturing a high resolution image of the droplet 606 in flight.
  • illumination source 610 for example comprising a strobed LED (Light Emitting Diode) 612 and a lens 614. Illumination is directed
  • Equipment 600 is controlled by a general purpose computer system 620 such as a personal computer into which have been installed a number of interface cards.
  • a print head drive card 620a interfaces with print head 602 and preferably allows the print head to be driven under similar conditions to those encountered during actual operation of the deposition process.
  • a GPIB (General Purpose Instrumentation Bus) interface card 620b drives a power supply 613 for strobed LED 612 to provide illumination in synchronism with drive the drive to the print head 602 such that droplet 606 is illuminated during its flight towards substrate 608.
  • An image acquisition card 620c captures digitized image frames from camera 618, and a local area network interface card 62Od will usually be present to interface with other computer systems, such as a printer (deposition) control computer system, to output drop volume measurement and/or calibration data.
  • Equipment 600 is preferably is fitted to the ink jet printer to facilitate calibration of the printer under close to operating conditions and at relatively frequent intervals.
  • X, Y, Z stage control for the print head/substrate have been omitted from Figure 5.
  • the print head drive signal typically comprises a unipolar or bipolar pulse drive, comprising a current pulse for a thermal (resistor-based) print head or a voltage pulse for a print head in which droplet ejection is driven by a piezoelectric transducer.
  • a print head When a print head is used to deposit a solution of material for fabricating an organic light emitting diode (OLED) it is observed that larger size droplets tend to have a long tail, which makes an accurate determination of their volume difficult. Smaller droplets tend to be more spherical so that once their area has been measured the presumption of a spherical shape, upon which a volume determination is predicated, is more likely to be correct, but it is more difficult to accurately measure the area of a smaller droplet in the first place.
  • One technique would be to characterize a print head nozzle using relatively low level print head drive signals to provide relatively small ejected droplets, and a graph of droplet volume against (usually) voltage drive has then been linearly extrapolated to larger level print head drive signals. The practice has been characterize one nozzle of the print head and then to assume that the same voltage-droplet volume function applies for all the nozzles.
  • a method of generating an improved nozzle driving signal for reducing a nozzle to nozzle volume variation of ink deposited on a substrate by a plurality of nozzles, the plurality of nozzles forming part of an ink jet print-head, and each nozzle depositing a volume of ink in response to a nozzle driving signal comprising: measuring the relative differences in the volume of ink deposited on the substrate from nozzle to nozzle when driving the nozzles with a known nozzle driving signal; determining an adjustment value dependent on a relationship between a change in the nozzle driving signal and a change in the volume of ink deposited on the substrate by the nozzles; and generating an improved nozzle driving signal using the measured variations in volume and the determined adjustment value, wherein the improved nozzle driving signal defines a nozzle driving signal that reduces the nozzle to nozzle volume variation of ink deposited on the substrate.
  • a method of determining a relationship between a change in a nozzle driving signal applied to a nozzle of an ink jet print-head and a change in a volume of ink deposited on a substrate the ink jet print- head comprising a plurality of nozzles, each nozzle depositing a volume of ink on a substrate in response to a nozzle driving signal, the method comprising driving selected nozzles from the plurality of nozzles with a first nozzle driving signal and driving the remaining nozzles from the plurality of nozzles with a second nozzle driving signal, the difference between the first and second nozzle driving signals representing a change in the nozzle driving signal; measuring the volume of ink deposited on the substrate by each of the plurality of nozzles; calculating a relative difference in a deposited volume of ink between the nozzles driven by the first nozzle driving signal and the nozzles driven by the second nozzle driving signal; and determining the relationship between the change in the nozzle driving signal and
  • the present invention also provides a method of printing a substrate with a reduced pixel to pixel volume variation, each pixel being formed by a volume of ink deposited on the substrate by an ink jet print-head, the print-head comprising a plurality of nozzles, each nozzle depositing a volume of ink in response to a nozzle driving signal, the method comprising: receiving a nozzle driving signal, the nozzle driving signal defining a plurality of driving parameters to drive the plurality of nozzles to obtain a desired deposition of ink on the substrate; retrieving adjustment data relating to each of the plurality of nozzles, the adjustment data defining an adjustment value for each of the plurality of nozzles in order to reduce a nozzle to nozzle variation in the volume of ink deposited by the plurality of nozzles; calculating an improved nozzle driving signal using the retrieved adjustment data and the nozzle driving signal, the improved nozzle driving signal defining a plurality of improved driving parameters of the plurality of nozzles to obtain the desired deposition of in
  • the present invention also provides a system for providing an improved nozzle driving signal to an ink jet print-head, the ink jet print-head comprising a plurality of nozzles, the plurality of nozzles depositing a volume of ink onto a substrate in response to a nozzle driving signal, the system comprising: an input for receiving the nozzle driving signal, the nozzle driving signal defining a plurality of driving parameters of the plurality of nozzles in order to obtain a desired deposition of ink on the substrate; retrieving means for retrieving adjustment data relating to each of the plurality of nozzles, the adjustment data defining an adjustment value; calculating means for calculating an improved nozzle driving signal; and an output to provide the improved nozzle driving signal to the ink jet print-head, wherein the calculating means calculates the improved nozzle driving signal using the retrieved adjustment data and the received nozzle driving signal, and wherein the improved nozzle driving signal defines a plurality of improved driving parameters of the plurality of nozzles to obtain the desired deposition of ink on the substrate
  • the above method is advantageous.
  • the adjustment data advantageously enables any substrate to be printed in any formation whilst achieving a reduced pixel to pixel volume variation.
  • the optimum nozzle driving signals are calculated based on the adjustment data such that there is no need to predict optimum driving signals beforehand.
  • the present invention also provides a data carrier carrying data for adjusting a plurality of nozzle driving signals for driving an inkjet print-head with a plurality of nozzles, the data comprising a plurality of nozzle drive adjustment values, each for adjusting the drive to a respective nozzle, each said adjustment value defining a relative adjustment of the drive of a nozzle with respect to the drive to one or more of the nozzles of the head.
  • processor control code such as conventional program code or code for setting up or controlling an ASIC (application specific integrated circuit) or FPGA (field programmable gate array).
  • This processor control code may be provided on a carrier medium such as a hard or floppy disk, CD- or DVD-rom, programmed memory such as read only memory (Firmware), or on a data carrier such as an optical or electrical signal carrier.
  • a carrier medium such as a hard or floppy disk, CD- or DVD-rom, programmed memory such as read only memory (Firmware), or on a data carrier such as an optical or electrical signal carrier.
  • Figures Ia and Ib show, respectively, cross sections through organic light emitting diode and a passive matrix OLED display
  • Figures 2a and 2b show, respectively, an ink jet printer and an ink jet printer head
  • Figures 3 a to 3 c show, respectively, conventional swathe printing, skewed printing for reduced dot pitch, and typical ink jet drop volume variations across a print head;
  • FIG. 4 shows a structure of a thin film transistor (TFT).
  • Figure 5 shows apparatus for measuring the volume of an ejected droplet of dissolved material during flight
  • Figure 6 shows the day-to-day velocity stability of a print head
  • Figure 7 shows the day-to-day volume stability of a print head
  • Figure 8 shows measurement repeatability of absolute volume measurements using a Zygo interferometer
  • Figure 9 shows substrates printed using adjusted nozzle driving signals
  • Figures 10a and 10b respectively, show substrates printed using adjusted nozzle driving signals of +5 V and -5 V;
  • Figure 11 shows a flow diagram of a method of generating an improved nozzle driving signal
  • Figure 12 shows results of the method of generating an improved nozzle driving signal according to the present invention
  • Figure 13 shows a substrate printed using the method according to the present invention.
  • variations in the nozzle-to-nozzle volume of ink deposited on a substrate by an ink jet print head can vary substantially across a print head due to manufacturing tolerances and other effects.
  • variations in the volume of ink will result in unwanted uneven emission characteristics across the device.
  • the signals used to drive an ink jet print head are provided as an image map, which defines a pattern of ink to deposit on the substrate.
  • the signals comprising the image map include, amongst others, nozzle driving signals to drive the nozzles and positioning data to position the print head over predefined areas of the substrate.
  • the nozzle driving signals comprise a voltage signal for each of the nozzles required to print at a particular location.
  • the magnitude and the duration of the signal define the velocity and volume of ink ejected from a particular nozzle in the print head.
  • Vprinthead V 1 -V 2
  • Figure 6 shows velocity measurements of a Dimatix SX3 ink jet print head using a Litrex 1408 printer, the printer having a drop analysis module velocity measurement system.
  • the print head was supplied with the same nozzle driving signal on each print run over the course of three days.
  • the velocity 'fingerprint' i.e. the relative nozzle-to-nozzle velocity of the ink droplets across the print head for the same nozzle driving signal, does not show the same shape at efach measurement.
  • the velocity measurements of some nozzles vary by more than 0.5m/s between the measurements.
  • Figure 7 shows volume measurements of ink deposited on a substrate by an ink jet print head using the same ink jet print head and printer as above.
  • the print head was supplied with the same nozzle driving signal on each of the print runs over the course of three days and the volume of ink deposited by each nozzle was measured using a Zygo New View 5000 series interferometer.
  • the print head and printer are the same as before.
  • the ink volume 'fingerprint' i.e. the relative nozzle-to-nozzle volume of the ink deposited on the substrate across the print head for the same nozzle driving signal, has substantially the same shape across the four readings (the variations in absolute volume measurement being within known repeatability error). Due to the stability of the deposited volume measurements over a period of time, it would therefore be preferable to calibrate for and control the deposited volume of the ink, instead of the ink drop velocity, in order to achieve the desired nozzle-to-nozzle volume variation.
  • Figure 8 shows volume measurements of a printed substrate for nozzles 0 to 52 in the above print head. The measurements were taken over a period of several days using the Zygo interferometer. It can be seen that the absolute volume measurements vary between readings, but the shape of the curves show the same trends, for example nozzle 39 has a low spike. The range of the 10 measurements is ⁇ 25 ⁇ m 3 across the print head. This represents a variation of absolute volume measurements (measured from the deposited ink) that is greater than ⁇ 2.1%. This indicates that the Zygo interferometer is not capable of absolute volume measurements to the accuracy required to determine the relationship between the change in nozzle driving signal and the change in the deposited ink volume. Neither is the Zygo interferometer accurate enough to achieve the target nozzle-to-nozzle volume control of ⁇ 1%.
  • the proposed method to determine the relationship between a change in a nozzle driving signal and a change in the volume of ink deposited on the substrate by the nozzles involves driving a selection of nozzles with a first nozzle driving signal, and driving the remaining nozzles in the print head with a second nozzle driving signal.
  • the difference between the first and second nozzle driving signals represents a change in the nozzle driving signal.
  • a measurement of the volume of ink deposited by each of the nozzles is then taken, and a relative difference between the volumes of ink deposited by the nozzles is calculated.
  • Substrates were printed using the above method.
  • every 5 th nozzle in the print head was driven with a voltage varying between — 5V and +5 V in IV increments. All of the remaining nozzles were driven using a fixed voltage.
  • other nozzles may be selected instead of every 5 th nozzle for this method.
  • the selection of nozzles is chosen such that a selection of odd and even numbered nozzles are driven using the varying signal.
  • FIGs 10a and 10b show a selection of the results from the above method.
  • plate 11 represents a base level print run of a substrate where every nozzle was supplied with the same nozzle driving signal, that is, the difference between the first and second nozzle driving signals is 0.
  • Plate 36 represents the print run where every 5 th nozzle was driven using a +5V signal, that is, the difference between the first and second nozzle driving signals was +5 V.
  • Figure 10b shows the same base level print run of the substrate plate 11 and plate 41, which represents the print run where every 5 th nozzle was driven using a -5V signal, that is, the difference between the first and second nozzle driving signals is -5V.
  • the volume measurements of every 5 th nozzle in the adjusted nozzle driving signals can then be mapped to the original print run, i.e. where no adjusted nozzle driving signal was used. From this mapping, a relationship between the change in nozzle driving signal and the change in the volume of ink deposited by the nozzles can be established using the volume offset for every 5 th nozzle.
  • the method effectively uses the relative difference in the measurements taken from different print runs in order to obviate the need for absolute volume measurements.
  • an improved nozzle driving signal can be generated in order to reduce the nozzle-to-nozzle volume variation of ink deposited on the substrate by the print head.
  • the method for generating the improved nozzle driving signal is described below.
  • a method of generating the improved nozzle driving signal is shown in figure 11.
  • a primary substrate is printed using a nozzle driving signal where each of the nozzles is driven using the same signal (1106), that is, the difference between the first and second nozzle driving signals is 0.
  • the volume of ink deposited on the substrate is then measured (1108) to determine the fingerprint for that particular print head, i.e. the nozzle-to-nozzle volume profile of the volume of ink deposited on the substrate.
  • the Zygo interferometer was again used to measure the volume of deposited ink, but other apparatus may be used instead.
  • the nozzle- driving signal used to print the substrate is stored for future reference (1112). However, if the nozzle-to-nozzle volume variation is not within the desired limit, the nozzle- driving signal is adjusted for each nozzle as required. The amount of adjustment is determined by the measurement of the relative difference between the volume of ink deposited by each nozzle and the known relationship between the change in nozzle- driving signal and the change in the volume of ink deposited on the substrate (1114). The aim of the adjustment is to level out the 'fingerprint', i.e.
  • a new substrate is then printed using the adjusted nozzle-driving signal (1116) and volume of ink deposited on the new substrate measured (1108). The process is repeated until the desired nozzle-to-nozzle volume variation is achieved.
  • Figure 12 shows a substrate printed using the above method.
  • the first substrate printed using the unadjusted nozzle-driving signal has a nozzle-to-nozzle volume variation that far exceeds the ⁇ 1% desired variation.
  • the nozzle-to-nozzle volume variation of ink deposited on the substrate is now within the desired ⁇ 1% level.
  • the number of iterations taken to achieve a nozzle-to-nozzle volume variation within the desired limits may be reduced by first calibrating the drop per nozzle velocity to a known value, typically 5m/s, before printing the primary substrate.
  • Figure 13 compares a substrates printed at the beginning of the method using the unadjusted nozzle-driving signal and a substrate printed using the improved nozzle- driving signal It can be seen that the substrate printed using the improved nozzle- driving signal has an improved emissions uniformity. It is important to note that when printing optical displays using ink jet techniques with improved emission characteristics, it is not the absolute value of the volume of ink deposited on the substrate that is the important factor, but the relative difference between the volume of the ink used to form the neighbouring pixels.
  • adjustment data generated from the above method may be stored in a memory device. This adjustment data may then be retrieved when necessary and an improved nozzle-driving signal calculated based on the desired deposition of ink and the adjustment data for the relevant nozzles.
  • the memory storage device may include, for example, ROM, RAM or other storage means, and is not limited in any way by these examples. No doubt many other effective alternatives will occur to the skilled person and it will be understood the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the scope of the claims appended hereto.

Abstract

A method of generating an improved nozzle driving signal for reducing a nozzle to nozzle volume variation of ink deposited on a substrate by an ink jet print-head, and each nozzle depositing a volume of ink in response to a nozzle driving signal, the method comprising: measuring the relative differences in the volume of ink deposited on the substrate from nozzle to nozzle when driving the nozzles with a known nozzle driving signal; determining an adjustment value dependent on a relationship between a change in the nozzle driving signal and a change in the volume of ink deposited on the substrate by the nozzles; and generating an improved nozzle driving signal using the measured variations in volume and the determined adjustment value, wherein the improved nozzle driving signal defines a nozzle driving signal that reduces the nozzle to nozzle volume variation of ink deposited on the substrate.

Description

DROPLET VOLUME CONTROL
This invention generally relates to the deposition of material for electronic devices, particularly molecular electronic devices such as organic light emitting diodes, by an ink jet-type process. The invention is particularly concerned with droplet volume measurement and calibration.
Organic light emitting diodes (OLEDs) comprise a particularly advantageous form of electro-optic display. They are bright, colourful, fast-switching, provide a wide viewing angle and are easy and cheap to fabricate on a variety of substrates. Organic LEDs may be fabricated using either polymers or small molecules in a range of colours (or in multi-coloured displays), depending upon the materials used. Examples of polymer- based organic LEDs are described in WO 90/13148, WO 95/06400 and WO 99/48160; examples of so-called small molecule based devices are described in US 4,539,507.
A basic structure 100 of a typical organic LED is shown in Figure Ia. The OLED 100 comprises a substrate 102, typically 0.7 mm or 1.1 mm glass but optionally clear plastic or some other substantially transparent material. An anode layer 104 is deposited on the substrate, typically comprising around 40 to 150 nm thickness of ITO (indium tin oxide), over part of which is provided a metal contact layer. Typically the contact layer comprises around 500nm of aluminium, or a layer of aluminium sandwiched between layers of chrome, and this is sometimes referred to as anode metal. Glass substrates coated with ITO and contact metal are widely available. The contact metal over the ITO helps provide reduced resistance pathways where the anode connections do not need to be transparent, in particular for external contacts to the device. The contact metal is removed from the ITO where it is not wanted, in particular where it would otherwise obscure the display, by a standard process of photolithography followed by etching. A substantially transparent hole injection layer 106 is deposited over the anode layer, followed by an electroluminescent layer 108, and a cathode 110. The electroluminescent layer 108 may comprise, for example, a PPV (poly(p- phenylenevinylene)) and the hole injection layer 106, which helps match the hole energy levels of the anode layer 104 and electroluminescent layer 108, may comprise a conductive transparent polymer, for example PEDOT:PSS (polystyrene-sulphonate- doped polyethylene-dioxythiophene) from H.C. Starck of Germany. In a typical polymer-based device the hole injection layer 106 may comprise around 200 nm of PEDOT. The light emitting polymer layer 108 is typically around 70 nm in thickness. These organic layers may be deposited by spin coating (afterwards removing material from unwanted areas by plasma etching or laser ablation) or by inkjet printing. In this latter case, banks 112 may be formed on the substrate, for example using photoresist, to define wells into which the organic layers maybe deposited. Such wells define light emitting areas or pixels of the display.
Cathode layer 110 typically comprises a low work function metal such as calcium or barium (for example deposited by physical vapour deposition) covered with a thicker, capping layer of aluminium. Optionally an additional layer may be provided immediately adjacent the electroluminescent layer, such as a layer of lithium fluoride, for improved electron energy level matching. Mutual electrical isolation of cathode lines may be achieved or enhanced through the use of cathode separators (not shown in Figure 1).
The same basic structure may also be employed for small molecule devices.
Typically a number of displays are fabricated on a single substrate and at the end of the fabrication process the substrate is scribed, and the displays separated before an encapsulating can is attached to each to inhibit oxidation and moisture ingress. Alternatively, the displays can be encapsulated prior to scribing and separating.
To illuminate the OLED, power is applied between the anode and cathode by, for example, battery 118 illustrated in Figure Ia. hi the example shown in Figure Ia light is emitted through transparent anode 104 and substrate 102 and the cathode is generally reflective. Such devices are referred to as "bottom emitters". Devices which emit through the cathode ("top emitters") may also be constructed, for example, by keeping the thickness of cathode layer 110 less than around 50-100 nm so that the cathode is substantially transparent and/or using a transparent cathode material such as ITO.
Referring now to Figure Ib, this shows a simplified cross-section through a passive matrix OLED display device 150, in which like elements to those of Figure 1 are indicated by like reference numerals. As shown, the hole injection layer 106 and the electroluminescent layer 108 are subdivided into a plurality of pixels 152 at the intersection of mutually perpendicular anode and cathode lines defined in the anode metal 104 and cathode layer 110 respectively. In the figure conductive lines 154 defined in the cathode layer 110 run into the page and a cross-section through one of a plurality of anode lines 158 running at right angles to the cathode lines is shown. An electroluminescent pixel 152 at the intersection of a cathode and anode line may be addressed by applying a voltage between the relevant lines. The anode metal layer 104 provides external contacts to the display 150 and may be used for both anode and cathode connections to the OLEDs (by running the cathode layer pattern over anode metal lead-outs).
The above mentioned OLED materials, and in particular the light emitting polymer material and the cathode, are susceptible to oxidation and to moisture. The device is therefore encapsulated in a metal or glass can 111, attached by UV-curable epoxy glue 113 onto anode metal layer 104. Preferably the anode metal contacts are thinned where they pass under the lip of the metal can 111 to facilitate exposure of glue 113 to UV light for curing.
It is known to deposit material for organic light emitting diodes (OLEDs) using ink jet printing techniques. This is described in, for example, T.R. Hebner, CC. Wu, D. Marcy, M.H. Lu and J.C. Sturm, "Ink-jet Printing of doped Polymers for Organic Light Emitting Devices", Applied Physics Letters, Vol. 72, No. 5, pp.519- 521, 1998; Y. Yang, "Review of Recent Progress on Polymer Electroluminescent Devices," SPIE Photonics West: Optoelectronics '98, Conf. 3279, San Jose, Jan., 1998; EP O 880 303; and "Ink- Jet Printing of Polymer Light-Emitting Devices", Paul C. Duineveld, Margreet M. de Kok, Michael Buechel, Aad H. Sempel, Kees A.H. Mutsaers, Peter van de Weijer, Ivo GJ. Camps, Ton J.M. van den Biggelaar, Jan-Eric J.M. Rubingh and Eliav I. Haskal, Organic Light-Emitting Materials and Devices V, Zakya H. Kafafi, Editor, Proceedings of SPIE Vol. 4464 (2002). InkJet techniques can be used to deposit materials for both small molecule and polymer LEDs, although these applications present their own particular problems, which are different to the problems encountered in conventional ink jet printing of images on paper or plastic, as will be explained more fully below. The use of the term "ink" in the following disclosure is taken to mean a dissolved molecular electronic material, which can include semi conductor material, Light Emitting Polymers (LEP) or small molecules.
Use of an ink jet printer to deposit red, green and blue colour filters for an electroluminescent display is described in EP l,219,980A. A similar technique is described in US 2002/0105688. Figures 2a and 2b, which are taken from EP 1,219,980, show ink jet printing apparatus which may be employed for this type of application. Figure 2a shows an ink jet printer 200 comprising a base 209 supporting first and second linear positioners 206, 208 for moving a substrate 212 and ink jet print head 222 relative to one another along two orthogonal axis Y and X. Positioner 206 comprises a pair of rails 254 mounting a slider 256 provided with a turntable 251 supporting a table or bed 249 on which the substrate 212 is supported. The substrate 212 is aligned on table or bed 249 by means of stops 250 against which two edges of the substrate abut. Turntable 251 allows the table and substrate 249, 212 to be rotated relative to the print head 222.
Positioner 208 comprises a pair of rails 252 mounting a slider 253 which carries rotary positioners 244, 246, 247 which allow a print head unit 226 carrying the print head to be rotated independently about three orthogonal axes. A further linear positioner 248 is also mounted on slider 253 to allow the print head unit and print head to be translated in the Z-direction, that is towards and away from substrate 212.
Ink jet printer system 200 is controlled by a computer terminal 202 via an umbilical 204. Terminal 202 may comprise a general purpose computer with interface hardware for interfacing to the above-described linear and rotary positioners, running operating system, user interface and other ink jet printer drive and control software, in a conventional manner. Thus terminal 202 typically includes a data input device such as a network interface of floppy disk drive for receiving data defining a pattern to be printed, and printer control software to control the printer hardware to print a pattern in accordance with stored or input data. Other conventional functions such as test functions, head cleaning functions and the like are generally also provided by software running on terminal 202.
Figure 2b shows print head 222 in more detail. The print head has a plurality of nozzles 227, typically orifices in a nozzle plate for ejecting droplets of fluid from the print head onto the substrate. A fluid supply for printing (not shown in Figure 2b) may either be provided by a reservoir within the print head or print head unit or fluid may be supplied from an external source. In the illustrated example the print head 222 has a single row 228 of nozzles 227, but in other examples of print heads more than one row of nozzles may be provided with nozzles offset in one or two dimensions. The diameter of the orifices of nozzles 227 is typically between lOμm and lOOμm, and drop sizes are similar. The space or pitch between adjacent nozzle orifices is typically between 50μm and 100/rni.
Figure 3 a shows a conventional printing strategy in which print head 222 prints successive swathes 302, 304 in the Y-direction, stepping in the X-direction between each swathe. The technique illustrated in Figure 3b may be employed to produce a finer dot pitch. The print head is positioned at an angle Φ to the X-direction to reduce the dot pitch by a factor of cos Φ. Figure 3c shows two examples 306 and 308 of the distribution of drop volume ejected from nozzles 227 across the width of print head 222. Generally the size or volume distribution of drops is non-uniform, increasing or falling off at nozzles at the edge of the print head (that is, near an end of a row of nozzles), and further non-uniformity arise from small variations in nozzle heights. Figure 3c shows variations in drop volume, but in general similar variations are also observed in drop velocity. This problem is sometimes addressed in conventional ink jet printing by overlapping the swathes 302, 304. When depositing materials for molecular electronic devices such as OLEDs, there is a need for both high resolution, generally than better than that required for the best high resolution graphics, and accurate control of the volume of material deposited. For graphics applications it is drop placement that is significant and volume variations of 5 to 10% are acceptable. However when constructing molecular electronic devices it is deposited "ink" volume which is important since this will determine the eventual film thickness which, for an OLED, impacts upon brightness and hence drive current and device lifetime. Thus it is desirable to achieve a volume variation of better than 2%, preferably better than 1%, across an entire OLED display.
To deposit a molecular electronic material a volatile solvent such as toluene or xylene is employed with 1-2% dissolved solvent material. This results in a relatively thin film in comparison with the initial "ink" volume. The drying time is dependent upon the solvent mix and the atmosphere above the substrate, but typically varies between a few seconds and some minutes. It is strongly preferable all the drops comprising material which are eventually to make up a pixel are deposited before drying begins. Solvents which may be used include alkylated benzenes, in particular toluene or xylene. Other solvents for inkjet printing are described in WO 00/59267, WO 01/16251 and WO 02/18513.
The pattern of material to be deposited is made up of pixels formed by depositing the electroluminescent material into a well (as described, for example, in EP 0 880 303) on a substrate. The wells are usually formed by photolithography of a photoresist as described in EP 0 862 156 to which reference may be made. In the case of OLEDs and other molecular electronic devices such as polymer FETs (Field Effect Transistors) these pixels and wells generally have regular shapes and a regular pattern, but in other cases the pixels can have irregular shapes. The substrate typically comprises a substantially non-absorbent material such as, for OLED displays, glass, clear plastics such as polyethylene or PET or other materials such as polyvinylidene fluoride or polyimide. In an OLED display the pixels are typically around 50μm wide and 40- 50μm long in a colour display or approximately three times this length in a monochrome display. The pixel spacing is typically 10-20μm. By contrast the print head is typically around lcm wide and a few centimetres long. InkJet printing processes may also be used in the creation of thin film transistors (TFT). An example structure of such a TFT is shown in Figure 4.
The TFT structure comprises a substrate 400 on which is deposited a gate electrode 402 followed by a dielectric layer 404 (for example, BCB (Benzocyclobutene); also inorganic materials such as SjOx or SjNx) and source and drain electrodes 406, 408. A layer of organic thin film transistor material 410, generally an organic semiconductor such as a polythiophene derivative is then deposited over the source and drain and dielectric layer.
InkJet printing processes are useable in at least the deposition of the organic semiconductor and dielectric materials.
One known strategy for more accurately controlling the volume of material deposited is to cover a pixel or fill a well using a plurality of sequentially deposited drops rather than a single drop, and this strategy is described in EP 1,219,980, in which the print head makes multiple passes in the Y-direction (referring to Figure 3a), depositing one drop onto a pixel on each pass. However this has the disadvantage that there is a relatively long period between successive drops landing on a single pixel, which can result in undesirable artefacts. Furthermore because a zig-zag scanning strategy is adopted for the X-direction the intervals between successive drops landing is non-uniform, depending upon the position of a pixel in the X-direction. With the technique described in EP 1,219,980 a slow drying solvent must be employed to prevent drying between successive swathes, but a greater flexibility in solvent choice is preferable and for some applications relatively quick drying solvents such as toluene and xylene, for example with drop drying times of the order of one second, are useful. The technique of EP 1,219,980 is directed towards averaging out drop landing errors (thus reducing "banding") as much as averaging out drop volume variations.
One technique for drop volume control is to calibrate one, or preferably a plurality of nozzles of a printhead by measuring the volume of an ejected drop whilst in flight for a range of printhead drive signals. Data collected in this way may then be used to determine or adjust a printhead drive signal in order to obtain a desired drop volume. Such a calibration procedure may be performed as part of a commissioning process for ink jet or droplet-based deposition apparatus, or a calibration procedure may be performed by the apparatus at switch on.
A problem with such a calibration procedure is that it is practically impossible to obtain an accurate determination of the volume of an ejected droplet of dissolved material. Figure 5 shows (not to scale) equipment 600 which may be employed to determine the volume of a droplet of dissolved material ejected from a droplet deposition head 602 of droplet deposition apparatus, such as ink jet print head of an ink jet-type printer, hi Figure 5 a droplet 606 of dissolved material has been ejected from a nozzle 604 of print head 602 and is in flight towards a substrate 608. Whilst droplet 606 is in flight it is illuminated from an illumination source 610, for example comprising a strobed LED (Light Emitting Diode) 612 and a lens 614. Illumination is directed at droplet 606 by means of a beam splitter 616 and droplet 606 is viewed through beam splitter 616 by a digital camera 618 capable of capturing a high resolution image of the droplet 606 in flight.
Equipment 600 is controlled by a general purpose computer system 620 such as a personal computer into which have been installed a number of interface cards. A print head drive card 620a interfaces with print head 602 and preferably allows the print head to be driven under similar conditions to those encountered during actual operation of the deposition process. A GPIB (General Purpose Instrumentation Bus) interface card 620b drives a power supply 613 for strobed LED 612 to provide illumination in synchronism with drive the drive to the print head 602 such that droplet 606 is illuminated during its flight towards substrate 608. An image acquisition card 620c captures digitized image frames from camera 618, and a local area network interface card 62Od will usually be present to interface with other computer systems, such as a printer (deposition) control computer system, to output drop volume measurement and/or calibration data. Equipment 600 is preferably is fitted to the ink jet printer to facilitate calibration of the printer under close to operating conditions and at relatively frequent intervals. The skilled person will appreciate that for clarity, other elements of the printer which are generally present, such as X, Y, Z stage control for the print head/substrate have been omitted from Figure 5.
In operation software running on general purpose computer system 620 controls the illumination and camera to capture a relatively high contrast image of a droplet in flight, corresponding to a known print head drive signal. The print head drive signal typically comprises a unipolar or bipolar pulse drive, comprising a current pulse for a thermal (resistor-based) print head or a voltage pulse for a print head in which droplet ejection is driven by a piezoelectric transducer. Once an image of an ejected droplet has been captured the volume of the droplet is determined by measuring the area or perimeter of the drop, assuming that the drop is spherical. In other arrangements two cameras may be employed to capture images of a droplet from two directions, preferably 90 degrees apart, to facilitate taking account of departure of a droplet from a spherical shape. Other illumination arrangements, such as fibre optic illumination, may also be employed.
In practice there are a number of difficulties in determining an accurate volume of a droplet in flight using the apparatus of Figure 5. The droplet is relatively small and is moving quickly (generally at several metres per second), and it is also difficult to provide good, even illumination. This tends to result in a relatively small depth of field for camera 618, which can result in out of focus images, and in practice it is also difficult to obtain a desired image contrast. Furthermore droplets are not necessarily spherical (as assumed) and these cumulative errors in droplet area measurement are then scaled up (by an additional power of length) when the volume of the droplet is determined.
When a print head is used to deposit a solution of material for fabricating an organic light emitting diode (OLED) it is observed that larger size droplets tend to have a long tail, which makes an accurate determination of their volume difficult. Smaller droplets tend to be more spherical so that once their area has been measured the presumption of a spherical shape, upon which a volume determination is predicated, is more likely to be correct, but it is more difficult to accurately measure the area of a smaller droplet in the first place. One technique would be to characterize a print head nozzle using relatively low level print head drive signals to provide relatively small ejected droplets, and a graph of droplet volume against (usually) voltage drive has then been linearly extrapolated to larger level print head drive signals. The practice has been characterize one nozzle of the print head and then to assume that the same voltage-droplet volume function applies for all the nozzles.
It will be recognized from the foregoing discussion that improved methods of driving an ink jet print head such that the nozzle-to-nozzle volume variation across the print head (and therefore across a OLED device printed using the print head) is minimised are desirable.
According to a first aspect of the present invention there is provided a method of generating an improved nozzle driving signal for reducing a nozzle to nozzle volume variation of ink deposited on a substrate by a plurality of nozzles, the plurality of nozzles forming part of an ink jet print-head, and each nozzle depositing a volume of ink in response to a nozzle driving signal, the method comprising: measuring the relative differences in the volume of ink deposited on the substrate from nozzle to nozzle when driving the nozzles with a known nozzle driving signal; determining an adjustment value dependent on a relationship between a change in the nozzle driving signal and a change in the volume of ink deposited on the substrate by the nozzles; and generating an improved nozzle driving signal using the measured variations in volume and the determined adjustment value, wherein the improved nozzle driving signal defines a nozzle driving signal that reduces the nozzle to nozzle volume variation of ink deposited on the substrate.
In another aspect of the present invention there is provided a method of determining a relationship between a change in a nozzle driving signal applied to a nozzle of an ink jet print-head and a change in a volume of ink deposited on a substrate, the ink jet print- head comprising a plurality of nozzles, each nozzle depositing a volume of ink on a substrate in response to a nozzle driving signal, the method comprising driving selected nozzles from the plurality of nozzles with a first nozzle driving signal and driving the remaining nozzles from the plurality of nozzles with a second nozzle driving signal, the difference between the first and second nozzle driving signals representing a change in the nozzle driving signal; measuring the volume of ink deposited on the substrate by each of the plurality of nozzles; calculating a relative difference in a deposited volume of ink between the nozzles driven by the first nozzle driving signal and the nozzles driven by the second nozzle driving signal; and determining the relationship between the change in the nozzle driving signal and the change in volume deposited on the substrate using the calculated difference in deposited volume of ink and the change in the nozzle driving signal.
The above method is advantageous. Broadly speaking, driving only a selection of nozzles with a first nozzle driving signal enables a measurement of the relative difference in the deposited ink from nozzle to nozzle. A relative measurement obviates the need for an accurate absolute volume measurement. Advantageously, this enables a less accurate measurement apparatus to be used whilst still enabling a result to be obtained.
The present invention also provides a method of printing a substrate with a reduced pixel to pixel volume variation, each pixel being formed by a volume of ink deposited on the substrate by an ink jet print-head, the print-head comprising a plurality of nozzles, each nozzle depositing a volume of ink in response to a nozzle driving signal, the method comprising: receiving a nozzle driving signal, the nozzle driving signal defining a plurality of driving parameters to drive the plurality of nozzles to obtain a desired deposition of ink on the substrate; retrieving adjustment data relating to each of the plurality of nozzles, the adjustment data defining an adjustment value for each of the plurality of nozzles in order to reduce a nozzle to nozzle variation in the volume of ink deposited by the plurality of nozzles; calculating an improved nozzle driving signal using the retrieved adjustment data and the nozzle driving signal, the improved nozzle driving signal defining a plurality of improved driving parameters of the plurality of nozzles to obtain the desired deposition of ink on the substrate with a reduced pixel to pixel volume variation; driving the plurality of nozzles using the improved nozzle driving signal in order to deposit ink on the substrate.
The present invention also provides a system for providing an improved nozzle driving signal to an ink jet print-head, the ink jet print-head comprising a plurality of nozzles, the plurality of nozzles depositing a volume of ink onto a substrate in response to a nozzle driving signal, the system comprising: an input for receiving the nozzle driving signal, the nozzle driving signal defining a plurality of driving parameters of the plurality of nozzles in order to obtain a desired deposition of ink on the substrate; retrieving means for retrieving adjustment data relating to each of the plurality of nozzles, the adjustment data defining an adjustment value; calculating means for calculating an improved nozzle driving signal; and an output to provide the improved nozzle driving signal to the ink jet print-head, wherein the calculating means calculates the improved nozzle driving signal using the retrieved adjustment data and the received nozzle driving signal, and wherein the improved nozzle driving signal defines a plurality of improved driving parameters of the plurality of nozzles to obtain the desired deposition of ink on the substrate with a reduced nozzle to nozzle volume variation of ink deposited on the substrate.
The above method is advantageous. The adjustment data advantageously enables any substrate to be printed in any formation whilst achieving a reduced pixel to pixel volume variation. The optimum nozzle driving signals are calculated based on the adjustment data such that there is no need to predict optimum driving signals beforehand.
The present invention also provides a data carrier carrying data for adjusting a plurality of nozzle driving signals for driving an inkjet print-head with a plurality of nozzles, the data comprising a plurality of nozzle drive adjustment values, each for adjusting the drive to a respective nozzle, each said adjustment value defining a relative adjustment of the drive of a nozzle with respect to the drive to one or more of the nozzles of the head.
The above described methods and apparatus may be implemented using processor control code such as conventional program code or code for setting up or controlling an ASIC (application specific integrated circuit) or FPGA (field programmable gate array). This processor control code may be provided on a carrier medium such as a hard or floppy disk, CD- or DVD-rom, programmed memory such as read only memory (Firmware), or on a data carrier such as an optical or electrical signal carrier. As the skilled person will appreciate such code may be distributed between a plurality of coupled components in communication with one another, for example across a network. These and other aspects of the invention can now be further described, by way of example only, and with reference to the accompanying drawings, in which:
Figures Ia and Ib show, respectively, cross sections through organic light emitting diode and a passive matrix OLED display;
Figures 2a and 2b show, respectively, an ink jet printer and an ink jet printer head;
Figures 3 a to 3 c show, respectively, conventional swathe printing, skewed printing for reduced dot pitch, and typical ink jet drop volume variations across a print head;
Figure 4 shows a structure of a thin film transistor (TFT);
Figure 5 shows apparatus for measuring the volume of an ejected droplet of dissolved material during flight;
Figure 6 shows the day-to-day velocity stability of a print head;
Figure 7 shows the day-to-day volume stability of a print head;
Figure 8 shows measurement repeatability of absolute volume measurements using a Zygo interferometer;
Figure 9 shows substrates printed using adjusted nozzle driving signals;
Figures 10a and 10b, respectively, show substrates printed using adjusted nozzle driving signals of +5 V and -5 V;
Figure 11 shows a flow diagram of a method of generating an improved nozzle driving signal; Figure 12 shows results of the method of generating an improved nozzle driving signal according to the present invention;
Figure 13 shows a substrate printed using the method according to the present invention.
As discussed above, variations in the nozzle-to-nozzle volume of ink deposited on a substrate by an ink jet print head can vary substantially across a print head due to manufacturing tolerances and other effects. When materials are deposited on a substrate to form OLED devices, variations in the volume of ink will result in unwanted uneven emission characteristics across the device.
Broadly speaking, we describe techniques to reduce the nozzle-to-nozzle volume variation of ink deposited on a substrate by an ink jet print head using measured data and stored adjustment data; and techniques to determine the adjustment data.
Generally, the signals used to drive an ink jet print head are provided as an image map, which defines a pattern of ink to deposit on the substrate. The signals comprising the image map include, amongst others, nozzle driving signals to drive the nozzles and positioning data to position the print head over predefined areas of the substrate. The nozzle driving signals comprise a voltage signal for each of the nozzles required to print at a particular location. The magnitude and the duration of the signal define the velocity and volume of ink ejected from a particular nozzle in the print head.
The effective voltage at the print head in this work is given as:
Vprinthead = V1-V2
As Vpπnthead is increased, ink drop velocity and volume increases. Therefore, as V2 increases, the ink volume will decrease.
As discussed above, it is often the practise to calibrate the print head to reduce the effect of such manufacturing tolerances by measuring and adjusting the velocity of the ink droplet being ejected from the nozzle. A number of nozzles are chosen and the velocity of the ejected ink measured to generate a relationship between the drive voltage and the velocity of the ink droplet. This relationship is then used to drive the remaining nozzles across the print head.
However, the applicants have discovered that this method does not produce nozzle-to- nozzle volume variations within the desired range of 2% and 1% across the print head. Furthermore, the velocity calibration appears only to remain within the desired limits for relatively short periods of time. As such, it is common to require a calibration step at the start of each day in order for the velocity measurements to remain within the desired limits.
Figure 6 shows velocity measurements of a Dimatix SX3 ink jet print head using a Litrex 1408 printer, the printer having a drop analysis module velocity measurement system. The print head was supplied with the same nozzle driving signal on each print run over the course of three days. As can be seen, the velocity 'fingerprint', i.e. the relative nozzle-to-nozzle velocity of the ink droplets across the print head for the same nozzle driving signal, does not show the same shape at efach measurement. In fact, there is an overall velocity reduction of ~025.m/s from the first measurement taken at time one of the first day and the subsequent measurement taken on the same day. The velocity measurements of some nozzles vary by more than 0.5m/s between the measurements.
Figure 7 shows volume measurements of ink deposited on a substrate by an ink jet print head using the same ink jet print head and printer as above. The print head was supplied with the same nozzle driving signal on each of the print runs over the course of three days and the volume of ink deposited by each nozzle was measured using a Zygo New View 5000 series interferometer. The print head and printer are the same as before. As can be seen, the ink volume 'fingerprint', i.e. the relative nozzle-to-nozzle volume of the ink deposited on the substrate across the print head for the same nozzle driving signal, has substantially the same shape across the four readings (the variations in absolute volume measurement being within known repeatability error). Due to the stability of the deposited volume measurements over a period of time, it would therefore be preferable to calibrate for and control the deposited volume of the ink, instead of the ink drop velocity, in order to achieve the desired nozzle-to-nozzle volume variation.
hi order to establish a set of improved nozzle driving signals, one must first establish the relationship between a change in nozzle driving signal and the change in the volume of ink deposited on the substrate. From this, an adjustment factor can be determined. Measurements of the volume of deposited ink are therefore needed for a range of drive voltages.
Figure 8 shows volume measurements of a printed substrate for nozzles 0 to 52 in the above print head. The measurements were taken over a period of several days using the Zygo interferometer. It can be seen that the absolute volume measurements vary between readings, but the shape of the curves show the same trends, for example nozzle 39 has a low spike. The range of the 10 measurements is ~25μm3 across the print head. This represents a variation of absolute volume measurements (measured from the deposited ink) that is greater than ±2.1%. This indicates that the Zygo interferometer is not capable of absolute volume measurements to the accuracy required to determine the relationship between the change in nozzle driving signal and the change in the deposited ink volume. Neither is the Zygo interferometer accurate enough to achieve the target nozzle-to-nozzle volume control of ±1%.
This is demonstrated in figure 9, where five substrates were printed with all nozzles across the print head using the same nozzle driving signal. V2 was increased from 37V to 41V in IV increments. As can be seen, although each nozzle driving signal shows the same fingerprint, i.e. the shape across the print head is substantially the same, it is not possible to find a relationship between the change in nozzle driving signal to change in volume due to the errors within the measurements. Clearly, measurement variations of greater than ±2.1% mean that a new method is necessary to determine the relationship between the change in nozzle driving signal and the change in volume of ink deposited on the substrate.
The proposed method to determine the relationship between a change in a nozzle driving signal and a change in the volume of ink deposited on the substrate by the nozzles involves driving a selection of nozzles with a first nozzle driving signal, and driving the remaining nozzles in the print head with a second nozzle driving signal. The difference between the first and second nozzle driving signals represents a change in the nozzle driving signal. A measurement of the volume of ink deposited by each of the nozzles is then taken, and a relative difference between the volumes of ink deposited by the nozzles is calculated.
Substrates were printed using the above method. In this method, every 5th nozzle in the print head was driven with a voltage varying between — 5V and +5 V in IV increments. All of the remaining nozzles were driven using a fixed voltage. Of course, other nozzles may be selected instead of every 5th nozzle for this method. Preferably, the selection of nozzles is chosen such that a selection of odd and even numbered nozzles are driven using the varying signal.
Figures 10a and 10b show a selection of the results from the above method. Turning first to figure 12a, plate 11 represents a base level print run of a substrate where every nozzle was supplied with the same nozzle driving signal, that is, the difference between the first and second nozzle driving signals is 0. Plate 36 represents the print run where every 5th nozzle was driven using a +5V signal, that is, the difference between the first and second nozzle driving signals was +5 V. Figure 10b shows the same base level print run of the substrate plate 11 and plate 41, which represents the print run where every 5th nozzle was driven using a -5V signal, that is, the difference between the first and second nozzle driving signals is -5V.
From these results, the volume measurements of every 5th nozzle in the adjusted nozzle driving signals can then be mapped to the original print run, i.e. where no adjusted nozzle driving signal was used. From this mapping, a relationship between the change in nozzle driving signal and the change in the volume of ink deposited by the nozzles can be established using the volume offset for every 5th nozzle. The method effectively uses the relative difference in the measurements taken from different print runs in order to obviate the need for absolute volume measurements.
By measuring different the volumes on different print runs and plotting the results, it was found that the nozzles in the print head exhibited a change in deposited ink volume of -5μm3 /Volt. Importantly, it was found that the odd and even numbered nozzles across the print head exhibited the same relationship.
Now that the relationship between the change in nozzle driving signal and the change in the volume of ink deposited on the substrate, an improved nozzle driving signal can be generated in order to reduce the nozzle-to-nozzle volume variation of ink deposited on the substrate by the print head. The method for generating the improved nozzle driving signal is described below.
A method of generating the improved nozzle driving signal is shown in figure 11. Once the relationship between the change in nozzle driving signal and the change in the volume of ink deposited on the substrate is known, a primary substrate is printed using a nozzle driving signal where each of the nozzles is driven using the same signal (1106), that is, the difference between the first and second nozzle driving signals is 0. The volume of ink deposited on the substrate is then measured (1108) to determine the fingerprint for that particular print head, i.e. the nozzle-to-nozzle volume profile of the volume of ink deposited on the substrate. The Zygo interferometer was again used to measure the volume of deposited ink, but other apparatus may be used instead.
If the nozzle-to-nozzle volume variation is already within the desired limits, the nozzle- driving signal used to print the substrate is stored for future reference (1112). However, if the nozzle-to-nozzle volume variation is not within the desired limit, the nozzle- driving signal is adjusted for each nozzle as required. The amount of adjustment is determined by the measurement of the relative difference between the volume of ink deposited by each nozzle and the known relationship between the change in nozzle- driving signal and the change in the volume of ink deposited on the substrate (1114). The aim of the adjustment is to level out the 'fingerprint', i.e. to reduce the nozzle-to- nozzle volume variation by increasing the volume of ink deposited by nozzles depositing relatively less ink and to reduce the volume of ink being deposited by nozzles depositing relatively more ink. A new substrate is then printed using the adjusted nozzle-driving signal (1116) and volume of ink deposited on the new substrate measured (1108). The process is repeated until the desired nozzle-to-nozzle volume variation is achieved.
Figure 12 shows a substrate printed using the above method. As can be seen, the first substrate printed using the unadjusted nozzle-driving signal has a nozzle-to-nozzle volume variation that far exceeds the ±1% desired variation. However, by using only two iterations of the above method, it can be seen that the nozzle-to-nozzle volume variation of ink deposited on the substrate is now within the desired ±1% level. The number of iterations taken to achieve a nozzle-to-nozzle volume variation within the desired limits may be reduced by first calibrating the drop per nozzle velocity to a known value, typically 5m/s, before printing the primary substrate.
Figure 13 compares a substrates printed at the beginning of the method using the unadjusted nozzle-driving signal and a substrate printed using the improved nozzle- driving signal It can be seen that the substrate printed using the improved nozzle- driving signal has an improved emissions uniformity. It is important to note that when printing optical displays using ink jet techniques with improved emission characteristics, it is not the absolute value of the volume of ink deposited on the substrate that is the important factor, but the relative difference between the volume of the ink used to form the neighbouring pixels.
To enable the printing of other substrates with a reduced pixel-to-pixel volume variation using an ink jet print head, adjustment data generated from the above method may be stored in a memory device. This adjustment data may then be retrieved when necessary and an improved nozzle-driving signal calculated based on the desired deposition of ink and the adjustment data for the relevant nozzles. The memory storage device may include, for example, ROM, RAM or other storage means, and is not limited in any way by these examples. No doubt many other effective alternatives will occur to the skilled person and it will be understood the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the scope of the claims appended hereto.

Claims

CLAIMS:
1. A method of generating an improved nozzle driving signal for reducing a nozzle to nozzle volume variation of ink deposited on a substrate by a plurality of nozzles, the plurality of nozzles forming part of an ink jet print-head, and each nozzle depositing a volume of ink in response to a nozzle driving signal, the method comprising: measuring the relative differences in the volume of ink deposited on the substrate from nozzle to nozzle when driving the nozzles with a known nozzle driving signal; determining an adjustment value dependent on a relationship between a change in the nozzle driving signal and a change in the volume of ink deposited on the substrate by the nozzles; and generating an improved nozzle driving signal using the measured variations in volume and the determined adjustment value, wherein the improved nozzle driving signal defines a nozzle driving signal that reduces the nozzle to nozzle volume variation of ink deposited on the substrate.
2. The method according to claim 1, wherein determining an adjustment value comprises: determining a relationship between a change in a nozzle driving signal and a change in the volume of ink deposited on the substrate by the nozzles; and determining the adjustment value for the plurality of nozzles using the determined relationship.
3. The method according to claim 2, wherein determining the relationship between a change in a nozzle driving signal and a change in the volume of ink deposited on the substrate by the nozzles comprises: driving a selection of the plurality of nozzles with a first nozzle driving signal and driving the remaining nozzles from the plurality of nozzles with a second nozzle driving signal, the difference between the first and second nozzle driving signals representing a change in the nozzle driving signal; measuring the volume of ink deposited on the substrate by each of the plurality of nozzles; calculating a relative difference in a deposited volume of ink between the nozzles driven by the first nozzle driving signal and the nozzles driven by the second nozzle driving signal; and determining the relationship between the change in nozzle driving signal and the change in volume deposited on the substrate using the calculated difference in deposited volume of ink and the change in the nozzle driving signal.
4. The method according to claim 3, wherein the selected nozzles are chosen to represent a selection of odd and even numbered nozzles from the plurality of nozzles.
5. The method according to claim 3 or 4, wherein every fifth nozzle from the plurality of nozzles is selected.
6. The method according to any preceding claim, wherein the nozzle to nozzle volume variation of ink deposited on the substrate using the improved driving signal is less than ±1%.
7. A method of driving an ink jet print-head to print ink onto a substrate, the print- head comprising a plurality of nozzles, each nozzle depositing a volume of ink on the substrate in response to a nozzle driving signal, the method comprising driving the ink jet print-head with the improved nozzle driving signal generated using the method of any preceding claim.
8. A method of determining a relationship between a change in a nozzle driving signal applied to a nozzle of an ink jet print-head and a change in a volume of ink deposited on a substrate, the ink jet print-head comprising a plurality of nozzles, each nozzle depositing a volume of ink on a substrate in response to a nozzle driving signal, the method comprising driving selected nozzles from the plurality of nozzles with a first nozzle driving signal and driving the remaining nozzles from the plurality of nozzles with a second nozzle driving signal, the difference between the first and second nozzle driving signals representing a change in the nozzle driving signal; measuring the volume of ink deposited on the substrate by each of the plurality of nozzles; calculating a relative difference in a deposited volume of ink between the nozzles driven by the first nozzle driving signal and the nozzles driven by the second nozzle driving signal; and determining the relationship between the change in the nozzle driving signal and the change in volume deposited on the substrate using the calculated difference in deposited volume of ink and the change in the nozzle driving signal.
9. The method according to claim 8, wherein the selected nozzles are chosen to represent a selection of odd and even numbered nozzles from the plurality of nozzles.
10. The method according to claim 8 or 9, wherein every fifth nozzle from the plurality of nozzles is selected.
11. The method according to any preceding claim, wherein the volume is measured using an interferometer.
12. A method of printing a substrate with a reduced pixel to pixel volume variation, each pixel being formed by a volume of ink deposited on the substrate by an ink jet print-head, the print-head comprising a plurality of nozzles, each nozzle depositing a volume of ink in response to a nozzle driving signal, the method comprising: receiving a nozzle driving signal, the nozzle driving signal defining a plurality of driving parameters to drive the plurality of nozzles to obtain a desired deposition of ink on the substrate; retrieving adjustment data relating to each of the plurality of nozzles, the adjustment data defining an adjustment value for each of the plurality of nozzles in order to reduce a nozzle to nozzle variation in the volume of ink deposited by the plurality of nozzles; calculating an improved nozzle driving signal using the retrieved adjustment data and the nozzle driving signal, the improved nozzle driving signal defining a plurality of improved driving parameters of the plurality of nozzles to obtain the desired deposition of ink on the substrate with a reduced pixel to pixel volume variation; driving the plurality of nozzles using the improved nozzle driving signal in order to deposit ink on the substrate.
13. The method according to claim 12, wherein the pixel to pixel volume variation when using the improved nozzle driving signal is less than ±1%.
14. The method according to any preceding claim, wherein the ink comprises a dissolved molecular electronic material.
15. The method according to claim 14 wherein the dissolved molecular electronic material comprises semiconductor material, light emitting polymers or small molecules.
16. A carrier medium carrying computer program code to, when running, implement the method of claim 12 or 13 or 14 or 15 when dependent on claim 12.
17. A system for providing an improved nozzle driving signal to an ink jet print- head, the ink jet print-head comprising a plurality of nozzles, the plurality of nozzles depositing a volume of ink onto a substrate in response to a nozzle driving signal, the system comprising: an input for receiving the nozzle driving signal, the nozzle driving signal defining a plurality of driving parameters of the plurality of nozzles in order to obtain a desired deposition of ink on the substrate; retrieving means for retrieving adjustment data relating to each of the plurality of nozzles, the adjustment data defining an adjustment value; calculating means for calculating an improved nozzle driving signal; and an output to provide the improved nozzle driving signal to the ink jet print-head, wherein the calculating means calculates the improved nozzle driving signal using the retrieved adjustment data and the received nozzle driving signal, and wherein the improved nozzle driving signal defines a plurality of improved driving parameters of the plurality of nozzles to obtain the desired deposition of ink on the substrate with a reduced nozzle to nozzle volume variation of ink deposited on the substrate.
18. The system according to claim 17, wherein the nozzle to nozzle volume variation of ink deposited on the substrate using the improved driving signal is less than ±1%.
19. The system according to claim 17 or 18, wherein the ink comprises a dissolved molecular electronic material.
20. The system according to claim 19 wherein the dissolved molecular electronic material comprises semiconductor material, light emitting polymers or small molecules.
21. A data carrier carrying data for adjusting a plurality of nozzle driving signals for driving an inkjet print-head with a plurality of nozzles, the data comprising a plurality of nozzle drive adjustment values, each for adjusting the drive to a respective nozzle, each said adjustment value defining a relative adjustment of the drive of a nozzle with respect to the drive to one or more of the nozzles of the head.
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