WO2000076777A1

WO2000076777A1 - Direct printing device

Info

Publication number: WO2000076777A1
Application number: PCT/SE1999/001070
Authority: WO
Inventors: Anders Ingelhag; Fredrik Blidner
Original assignee: Array Ab
Priority date: 1999-06-16
Filing date: 1999-06-16
Publication date: 2000-12-21
Also published as: AU5073499A

Abstract

The present invention relates to a printhead structure for use in direct electrostatic printing devices for modulating a transport of charged particles from a particle source toward a back electrode member. The printhead structure includes a substrate of electrically insulating material and a pattern of control electrodes arranged on a surface of the substrate. Apertures are made through the substrate for enabling the transport of charged particles, each aperture being controlled by corresponding control electrode. The apertures are made by delivering radiation pulses onto the substrate whereby the incident energy is absorbed in the substrate material which is heated and ablated. Residual amount of melted material contaminating the aperture walls after formation of the apertures, are effectively released and volatized in a gas plasma treatment in order to ensure a uniform surface conductivity within the apertures, thereby improving the dot position control.

Description

Direct Printing Device Field of the Invention

The invention relates to a direct printing apparatus in which a computer generated image information is converted into a pattern of electrostatic fields, which selectively transport electrically charged particles from a particle source toward a back electrode through a printhead structure, whereby the charged particles are deposited in image configuration on an image receiving substrate caused to move relative to the printhead structure. More specifically, the invention relates to an improved printhead structure which allows higher print quality and print uniformity. The invention also relates to a improved method for manufacturing such a printhead structure.

Background of the Invention

U.S Patent No. 5,036,341 discloses a direct electrostatic printing device and a method to produce text and pictures with toner particles on an image receiving substrate directly from computer generated signals. Such a device generally includes a printhead structure provided with a plurality of apertures through which toner particles are selectively transported from a particle source to an image receiving medium due to control in accordance with an image information.

A printhead structure for use in such a method includes generally a thin, flexible, sheet-like substrate of electrically insulating polymer material, such as, for example, polyimide or the like. The substrate is provided with a plurality of apertures, having generally a circular shape with a diameter in a range of, for example 100 to 200 micrometer. The apertures are generally obtained utilizing excimer laser micromachining methods in which UV radiation is delivered on the surface of the substrate at repetition rate up to 100 Hz, whereby the incident energy is absorbed in a thin layer, which is rapidly decomposed, heated and ablated. Each incident laser pulse removes a well defined thin layer of polymer material so that depth control of the aperture can be very exact. One of the main advantages of excimer laser micromachining techniques is that they can be used in a mask projection mode to transfer a complex pattern onto the substrate, allowing an exact shape control of the apertures . A drawback of excimer laser micromachining methods is that the substrate needs to be protected from debris generated during the ablation process. Though most of the material ablated by the excimer laser is vaporized, some falls back onto the substrate or onto the aperture walls as a hot, carbonaceous slag that melts into the polymer material. Generally, the slag deposit has a higher conductivity than the substrate material as a whole, which may result in a non-uniform charge distribution on the aperture wall surface. This, in turn, may degrade the print uniformity since the toner stream through an aperture tends to be slightly deflected from its initial trajectory due to interaction with residual charge deposits on the aperture walls . Residual material deposits from the ablation process can influence the print uniformity due to asymmetric charge density on the aperture walls and due to the amount of residual charge contaminating each particular aperture. As a result, the printed dots will be slightly displaced from their intended positions, and that displacement will vary from one aperture to another depending of the distribution of slag deposit onto the aperture walls. Furthermore, the amount of toner particles allowed to pass through a particular aperture will also depend on the surface conductivity of the aperture walls, i.e. the dot density will vary from one aperture to another due to the amount of slag deposit onto the aperture walls. That is, the apertures are liable to "behave" differently with respect to both dot position and dot density. Therefore, in order to meet the requirements of higher print uniformity in direct electrostatic printing devices and methods, there is still a need for an improved printhead structure and improved manufacturing methods for producing such a printhead structure. Summary of the invention The present invention relates to a printhead structure for use in a direct electrostatic image forming apparatus, and a manufacturing method for producing such a printhead structure. A printhead structure in accordance with the present invention is used in a direct electrostatic printing device for modulating a transport of charged particles from a particle source toward a back electrode member, the transported charged particles thereby being deposited at predetermined dot positions on an image receiving member interposed between the printhead structure and the back electrode member. The printhead structure includes a substrate of electrically insulating material having a predetermined thickness, a first surface facing the particle source and a second surface facing the back electrode member; a first pattern of conductive elements arranged on the first surface of the substrate, including a plurality of control electrodes; control voltage sources connected to the control electrodes for selectively controlling the transport of charged particles; and a plurality of apertures arranged through the printhead structure for enabling the transport of charged particles from the particle source toward the back electrode member, each aperture thus having a surface extending in the thickness of the substrate.

The aperture surfaces are cleaned in a gas plasma treatment during which undesired variations of the surface conductivity of the aperture surfaces are reduced in order to eliminate unpredictable charge interaction between the transported charged particles and the aperture surfaces, thereby reducing dot position error.

It can be shown that the dot position error, defined as the amount by which obtained dots diverge from their predetermined locations due to undesired variations of the surface conductivity of the aperture surfaces, is considerably reduced by the gas plasma treatment.

The invention also relates to a method for producing a printhead structure for use in a direct electrostatic printing device to modulate a transport of charged particles from a particle source toward an image receiving member, the method including the steps of: providing a substrate of electrically insulating material, forming a pattern of conductive elements, including a plurality of control electrodes, on a first surface of said substrate determining a plurality of target positions on said first surface of the substrate, delivering radiation pulses on the target locations for heating and ablating the substrate material to form apertures through the substrate exposing said apertures to a gas plasma treatment to remove residual amounts of said heated and ablated substrate material from the aperture surfaces; and coating at least a part of the substrate with a coverlayer of electrically insulating material, The gas plasma treatment is performed in a reaction chamber containing a gas plasma which is caused to react with the substrate material on the aperture surfaces, so as to volatilize residual amounts of said heated and ablated substrate material.

Brief description of the drawings

The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention and wherein: Fig.l is a schematic view of an image forming apparatus for direct electrostatic printing.

Fig.2 is a schematic section view across a print station in an image forming apparatus as that shown in Fig.l. Fig.3 is a schematic section view of the print zone, illustrating the positioning of a printhead structure in relation to a particle source and an image receiving member .

Fig. a is a partial view of a printhead structure, showing the surface of the printhead structure that is facing the particle source.

Fig.4b is a partial view of a printhead structure, showing the surface of the printhead structure that is facing the image receiving member. Fig.4c is a section view across a section line I-I in the printhead structure of Fig.4a and across the corresponding section line II-II of Fig.4b.

Detailed description of the embodiments

To perform a direct electrostatic printing method in accordance with the present invention, a background electric field is produced between a particle carrier and a back electrode to enable a transport of charged particles therebetween. A printhead structure, such as an electrode matrix provided with a plurality of selectable apertures, is interposed in the background electric field between the particle carrier and the back electrode and connected to a control unit which converts the image information into a pattern of electrostatic fields which, due to control in accordance with the image information, selectively open or close passages in the electrode matrix to permit or restrict the transport of charged particles from the particle carrier. The modulated stream of charged particles allowed to pass through the opened apertures are thus exposed to the background electric field and propelled toward the back electrode. The charged particles are deposited on the image receiving substrate to provide line-by line scan printing to form a visible image.

A printhead structure for use in direct electrostatic printing may take on many designs, such as a lattice of intersecting wires arranged in rows and columns, or an apertured substrate of electrically insulating material overlaid with a printed circuit of control electrodes arranged in conjunction with the apertures. Generally, a printhead structure includes a flexible substrate of insulating polymer material such as polyimide or the like, having a first surface facing the particle carrier, a second surface facing the back electrode and a plurality of apertures arranged through the substrate. The first surface is coated with printed circuit comprising control electrodes, each of which surrounds a corresponding aperture. The control electrodes preferably has a ring shaped structure or any other suitable design having symmetry about a central axis of the aperture through the substrate.

The apertures are preferably aligned in one or several rows extending transversally across the width of the substrate, i.e. perpendicular to the motion direction of the image receiving substrate.

According to such a method, each single aperture is utilized to address a specific pixel location of the image in a transversal direction. Thus, the transversal print addressability is limited by the density of apertures through the printhead structure. For instance, a print addressability of 300 dpi requires a printhead structure having 300 apertures per inch in a transversal direction.

According to a preferred embodiment of the present invention, a direct electrostatic printing device includes a dot deflection control (DDC) . According to that embodiment, each single aperture is used to address several pixel locations on an image receiving member by controlling not only the transport of toner particles through the aperture, but also their transport trajectory toward the image receiving substrate, and thereby the location of the obtained dot. The DDC method increases the print addressability without requiring a larger number of apertures in the printhead structure. This is achieved by providing the printhead structure with deflection electrodes connected to variable deflection voltages which, during each print cycle, sequentially modify the radial symmetry of the electrostatic control fields about the central axis of the aperture, in order to deflect the modulated stream of toner particles in predetermined deflection directions. For instance, a DDC method performing three deflection steps per print cycle, provides a print addressability of 600 dpi utilizing a printhead structure having only 200 apertures per inch. According to a preferred embodiment, an improved DDC method provides a simultaneous dot size and dot position control. This method utilizes the deflection electrodes to influence the convergence of the modulated stream of toner particles thus controlling the dot size. Each aperture is surrounded by two deflection segments connected to respective deflection voltages Dl , D2 , such that the electrostatic control field generated by the control electrode remains substantially symmetrical as long as both deflection voltages Dl , D2 have the same amplitude. The amplitude of Dl and D2 are modulated to apply converging forces on toner particles as they are transported toward the image receiving member, thus providing smaller dots. The dot position is simultaneously controlled by modulating the amplitude difference between Dl and D2 to deflect the toner trajectory toward predetermined pixel locations. A printhead structure for use in DDC methods further includes a second printed circuit, preferably arranged on the second surface of the substrate, and comprising a plurality of deflection electrodes. The deflection electrodes are supplied with deflection voltages Dl , D2 in subsequent steps corresponding to predetermined deflection modes. Utilizing such a method, 60 micrometer dots can be obtained with apertures having a diameter in the order of 160 micrometer. In order to clarify the method and device according to the invention, some examples of its use will now be described in connection with accompanying drawings . As shown in Fig.l, an image forming apparatus in accordance with the present invention comprises at least one print station, preferably four print stations (Y, M, C, K) , an intermediate image receiving member 1, a driving roller 10, at least one support roller 11, and preferably several adjustable holding elements 12. The four print stations are arranged in relation to the intermediate image receiving member 1. The image receiving member, preferably a transfer belt 1 is mounted over the driving roller 10. The at least one support roller 11 is provided with a mechanism for maintaining the transfer belt 1 with a constant tension, while preventing transversal movement of the transfer belt 1. The holding elements 12 are for accurately positioning the transfer belt 1 with respect to each print station. The driving roller 10 is preferably a cylindrical metallic sleeve having a rotation axis extending perpendicular to the motion direction of the belt 1 and a rotation velocity adjusted to convey the belt 1 at a velocity of one addressable dot location per print cycle, to provide line by line scan printing. The adjustable holding elements 12 are arranged for maintaining the surface of the belt at a predetermined gap distance from each print station. The holding elements 12 are preferably cylindrical sleeves disposed perpendicularly to the belt motion in an arcuated configuration so as to slightly bend the belt 1 at least in the vicinity of each print station in order to, in combination with the belt tension, create a stabilization force component on the belt. That stabilization force component is opposite in direction and preferably larger in magnitude than an electrostatic attraction force component acting on the belt 1 due to interaction with the different electric potentials applied on the corresponding print station. The transfer belt 1 is preferably an endless band of 30 to 200 microns thick composite material as a base. The base composite material can suitably include thermoplastic polyamide resin or any other suitable material having a high thermal resistance, such as 260°C of glass transition point and 388°C of melting point, and stable mechanical properties under temperatures in the order of 250°C. The composite material of the transfer belt has preferably a homogeneous concentration of filler material, such as carbon or the like, which provides a uniform electrical conductivity throughout the entire surface of the transfer belt 1. The outer surface of the transfer belt 1 is preferably coated with a 5 to 30 microns thick coating layer made of electrically conductive polymer material having appropriate conductivity, thermal resistance, adhesion properties, release properties and surface smoothness . The transfer belt 1 is conveyed past the four different print stations, whereas toner particles are deposited on the outer surface of the transfer belt and superposed to form a four color toner image. Toner images are then preferably conveyed through a fuser unit 13 comprising a fixing holder 14 arranged transversally in direct contact with the inner surface of the transfer belt. The fixing holder includes a heating element 15 preferably of a resistance type of e.g. molybdenium, maintained in contact with the inner surface of the transfer belt 1. As an electric current is passed through the heating element 15, the fixing holder 14 reaches a temperature required for melting the toner particles deposited on the outer surface of the transfer belt 1. The fusing unit 13 further includes a pressure roller 16 arranged transversally across the width of the transfer belt 1 and facing the fixing holder 14. An information carrier 2, such as a sheet of plain untreated paper or any other medium suitable for direct printing, is fed from a paper delivery unit 21 and conveyed between the pressure roller 16 and the transfer belt. The pressure roller 16 rotates with applied pressure to the heated surface of the fixing holder 14 whereby the melted toner particles are fused on the information carrier 2 to form a permanent image. After passage through the fusing unit 13, the transfer belt is brought in contact with a cleaning element 17, such as for example a replaceable scraper blade of fibrous material extending across the width of the transfer belt 1 for removing all untransferred toner particles from the outer surface. As shown in Fig.2, a print station in an image forming apparatus in accordance with the present invention includes a particle delivery unit 3 preferably having a replaceable or refillable container 30 for holding toner particles, the container 30 having front and back walls (not shown) , a pair of side walls and a bottom wall having an elongated opening 31 extending from the front wall to the back wall and provided with a toner feeding element 32 disposed to continuously supply toner particles to a developer sleeve 33 through a particle charging member 34. The particle charging member 34 is preferably formed of a supply brush or a roller made of or coated with a fibrous, resilient material. The supply brush is brought into mechanical contact with the peripheral surface of the developer sleeve 33 for charging particles by contact charge exchange due to triboelectrification of the toner particles through frictional interaction between the fibrous material on the supply brush and any suitable coating material of the developer sleeve. The developer sleeve 33 is preferably made of metal coated with a conductive material, and preferably has a substantially cylindrical shape and a rotation axis extending parallel to the elongated opening 31 of the particle container 30. Charged toner particles are held to the surface of the developer sleeve 33 by electrostatic forces essentially proportional to (Q/D)², where Q is the particle charge and D is the distance between the particle charge center and the boundary of the developer sleeve 33. Alternatively, the charge unit may additionally include a charging voltage source (not shown) , which supply an electric field to induce or inject charge to the toner particles. Although it is preferred to charge particles through contact charge exchange, the method can be performed using any other suitable charge unit, such as a conventional charge injection unit, a charge induction unit or a corona charging unit, without departing from the scope of the present invention.

A metering element 35 is positioned proximate to the developer sleeve 33 to adjust the concentration of toner particles on the peripheral surface of the developer sleeve 33, to form a relatively thin, uniform particle layer thereon. The metering element 35 may be formed of a flexible or rigid, insulating or metallic blade, roller or any other member suitable for providing a uniform particle layer thickness. The metering element 35 may also be connected to a metering voltage source (not shown) which influence the triboelectrification of the particle layer to ensure a uniform particle charge density on the surface of the developer sleeve. As shown in Fig.3, the developer sleeve 33 is arranged in relation with a positioning device 40 for accurately supporting and maintaining the printhead structure 5 in a predetermined position with respect to the peripheral surface of the developer sleeve 33. The positioning device 40 is formed of a frame 41 having a front portion, a back portion and two transversally extending side rulers 42, 43 disposed on each side of the developer sleeve 33 parallel with the rotation axis thereof. The first side ruler 42 , positioned at a upstream side of the developer sleeve 33 with respect to its rotation direction, is provided with fastening means 44 to secure the printhead structure 5 along a transversal fastening axis extending across the entire width of the printhead structure 5. The second side ruler 43, positioned at a downstream side of the developer sleeve 33, is provided with a support element 45, or pivot, for supporting the printhead structure 5 in a predetermined position with respect to the peripheral surface of the developer sleeve 33. The support element 45 and the fastening axis are so positioned with respect to one another, that the printhead structure 5 is maintained in an arcuated shape along at least a part of its longitudinal extension. That arcuated shape has a curvature radius determined by the relative positions of the support element 45 and the fastening axis and dimensioned to maintain a part of the printhead structure 5 curved around a corresponding part of the peripheral surface of the developer sleeve 33. The support element 45 is arranged in contact with the printhead structure 5 at a fixed support location on its longitudinal axis so as to allow a slight variation of the printhead structure 5 position in both longitudinal and transversal direction about that fixed support location, in order to accommodate a possible excentricity or any other undesired variations of the developer sleeve 33. That is, the support element 45 is arranged to made the printhead structure 5 pivotable about a fixed point to ensure that the distance between the printhead structure 5 and the peripheral surface of the developer sleeve 33 remains constant along the whole transverse direction at every moment of the print process, regardless of undesired mechanical imperfections of the developer sleeve 33. The front and back portions of the positioning device 40 are provided with securing members 46 on which the toner delivery unit 3 is mounted in a fixed position to provide a constant distance between the rotation axis of the developer sleeve 33 and a transversal axis of the printhead structure 5. Preferably, the securing members 46 are arranged at the front and back ends of the developer sleeve 33 to accurately space the developer sleeve 33 from the corresponding holding element 12 of the transfer belt 1 facing the actual print station. The securing members 46 are preferably dimensioned to provide and maintain a parallel relation between the rotation axis of the developer sleeve 33 and a central transversal axis of the corresponding holding member 12.

As shown in Fig.4a, 4b, 4c, a printhead structure 5 in an image forming apparatus in accordance with the present invention comprises a substrate 50 of flexible, electrically insulating material such as polyimide or the like, having a predetermined thickness, a first surface facing the developer sleeve, a second surface facing the transfer belt, a transversal axis 51 extending parallel to the rotation axis of the developer sleeve 33 across the whole print area, and a plurality of apertures 52 arranged through the substrate 50 from the first to the second surface thereof. The first surface of the substrate is overlaid with a first printed circuit, comprising a plurality of control electrodes 53 disposed in conjunction with the apertures, and, in some embodiments, shield electrode structures (not shown) arranged in conjunction with the control electrodes 53. The second surface of the substrate is coated with a second cover layer 502 of electrically insulating material, such as for example parylene. A second printed circuit, including a plurality of deflection electrodes 54, is arranged between the substrate 50 and the second cover layer 502. The printhead structure 5 further includes a layer of antistatic material (not shown) , preferably a semiconductive material, such as silicium oxide or the like, arranged on at least a part of the second cover layer 502, facing the transfer belt 1. The printhead structure 5 is brought in cooperation with a control unit (not shown) comprising variable control voltage sources connected to the control electrodes 53 to supply control potentials which control the amount of toner particles to be transported through the corresponding aperture 52 during each print sequence. The control unit further comprises deflection voltage sources (not shown) connected to the deflection electrodes 54 to supply deflection voltage pulses which controls the convergence and the trajectory path of the toner particles allowed to pass through the corresponding apertures 52. The control unit, in some embodiments, even includes a shield voltage source (not shown) connected to the shield electrodes to supply a shield potential which electrostatically screens adjacent control electrodes 53 from one another, preventing electrical interaction therebetween. In a preferred embodiment of the invention, the substrate 50 is a flexible sheet of polyimide having a thickness on the order of about 50 microns. The first and second printed circuits are copper circuits of approximately 8-9 microns thickness etched onto the first and second surface of the substrate 50, respectively, using conventional etching techniques. The first and second cover layers (501, 502) are 5 to 10 microns thick parylene laminated onto the substrate 50 using vacuum deposition techniques. The apertures 52 are made through the printhead structure 5 using conventional laser micromachining methods. The apertures 52 have preferably a circular or elongated shape centered about a central axis, with a diameter in a range of 80 to 120 microns, alternatively a transversal minor diameter of about 80 microns and a longitudinal major diameter of about 120 microns. Although the apertures 52 have preferably a constant shape along their central axis, for example cylindrical apertures, it may be advantageous in some embodiments to provide apertures whose shape varies continuously or stepwise along the central axis, for example conical apertures.

In a preferred embodiment of the present invention, the printhead structure 5 is dimensioned to perform 600 dpi printing utilizing three deflection sequences in each print cycle, i.e. three pixel locations are addressable through each aperture 52 of the printhead structure during each print cycle. Accordingly, one aperture 52 is provided for every third pixel location in a transverse direction, that is, 200 equally spaced apertures per inch aligned parallel to the transversal axis 51 of the printhead structure 5. The apertures 52 are generally aligned in one or several rows , preferably in two parallel rows each comprising 100 apertures per inch. Hence, the aperture pitch, i.e. the distance between the central axes of two neighbouring apertures of a same row is 0,01 inch or about 254 microns. The aperture rows are preferably positioned on each side of the transversal axis 51 of the printhead structure 5 and transversally shifted with respect to each other such that all apertures are equally spaced in a transverse direction. The distance between the aperture rows is preferably chosen to correspond to a whole number of pixel locations .

The first printed circuit comprises control electrodes 53 each of which having a ring-shaped structure surrounding the periphery of a corresponding aperture 52, and a connector preferably extending in the longitudinal direction, connecting the ring shaped structure to a corresponding control voltage source. Although a ring- shaped structure is preferred, the control electrodes 53 may take on various shape for continuously or partly surrounding the apertures 52, preferably shapes having symmetry about the central axis of the apertures. In some embodiments, particularly when the apertures 52 are aligned in one single row, the control electrodes are advantageously made smaller in a transverse direction than in a longitudinal direction.

The second printed circuit comprises a plurality of deflection electrodes 54, each of which is divided into two semicircular or crescent shaped deflection segments 541, 542 spaced around a predetermined portion of the circumference of a corresponding aperture 52. The deflection segments 541, 542 are arranged symmetrically about the central axis of the aperture 52 on each side of a deflection axis 543 extending through the center of the aperture 52 at a predetermined deflection angle d to the longitudinal direction. The deflection axis 543 is dimensioned in accordance with the number of deflection sequences to be performed in each print cycle in order to neutralize the effects of the belt motion during the print cycle, to obtain transversally aligned dot positions on the transfer belt. For instance, when using three deflection sequences, an appropriate deflection angle is chosen to arctan(l/3), i.e. about 18,4°. Accordingly, the first dot is deflected slightly upstream with respect to the belt motion, the second dot is undeflected and the third dot is deflected slightly downstream with respect to the belt motion, thereby obtaining a transversal alignment of the printed dots on the transfer belt. Accordingly, each deflection electrode 54 has a upstream segment 541 and a downstream segment 542, all upstream segments 541 being connected to a first deflection voltage source Dl , and all downstream segments 542 being connected to a second deflection voltage source D2. Three deflection sequences (for instance: Dl<D2 ; D1=D2; D1>D2) can be performed in each print cycle, whereby the difference between Dl and D2 determines the deflection trajectory of the toner stream through each aperture 52, thus the dot position on the toner image.

An essential requirement of direct electrostatic printing is that each pixel location can be addressed correctly throughout the entire print area. However, even though the deflection length of each deflected dot is exactly dimensioned to provide an accurate dot position control adapted to high resolution printing, a slight divergence from the intended pixel location may still occur due to undesired variation of the surface conductivity on the aperture walls or on the surrounding of each orifice. An object of the present invention is to provide a printhead structure in which all apertures have the same surface conductivity, thereby eliminating or at least considerably reducing a dot position error. The dot position error, referred hereinafter as the amount by which a obtained dot diverges from its intended position on the transfer belt due to surface conductivity variations within an aperture, is caused by residual amounts of melted material that contaminate the aperture walls and interact with the transported charged particles. When laser pulses are delivered on the substrate to form the apertures, the incident energy is absorbed in the substrate material, which is melted and partly volatilized. The melted part remaining on the wall surface have modified electrical properties compared with the substrate material itself. As that melted residues are generally spread in an unpredictable way around the aperture surfaces, they tend to modify the symmetry of the field configuration within the aperture, i.e. to cause an uncontrolled deviation of the particle transport. That deviation may result in a dot position error of 50 microns or more, depending on the distance between the aperture and the transfer belt. As that distance is generally in the order of 1 mm, even the slightest deviation initiated within the aperture can drastically degrade the dot position control. To ensure a sufficient accuracy of dot position control in 600 dpi printing, wherein the distance between two neighbouring pixel locations is only about 42 microns, it is therefore necessary to release all residual melted debris from the aperture walls after forming the apertures. For instance, the uniformity of a printhead structure can be determined by a ratio of the average dot position error and the dot pitch. In a 600 dpi resolution mode, a tolerance of 50% requires an average dot error less than 21 microns. Tolerances of 50% or less can be obtained only by ensuring a uniform surface conductivity in the aperture walls, i.e. ensuring a uniform surface conductivity within each single aperture and eliminating variations in surface conductivity from one aperture to another. It has been observed that those requirements are effectively reached using a gas plasma cleaning process, in which the surface material of the aperture walls is caused to react, both chemically and physically, with a gas plasma. The plasma reacts with the surface material of the aperture walls by forming or breaking chemical bonds, or by energetic ion bombardment, thereby volatizing carbonaceous debris originated from the ablation process .

A gas plasma suitable for a cleaning process according to the present invention, contains preferably oxygen, for instance in the form of 0⁺ and 0²⁺ ions, and small amounts of tetrafluormethane (CF₄) or noble gases, such as Argon.

The present invention also relates to a manufacturing method in which the aforementioned gas plasma treatment has been included. The method is described in the following embodiment which is given only as an illustrative example and can not be considered as limitative for the present invention.

Provided a flexible substrate of electrically insulating polymer material, such as Upilex®, a manufacturing method for producing a printhead structure according to a first embodiment of the invention, comprises the following main steps :

1. the etching process, in which copper patterns, comprising the control and deflection electrodes are formed on both side of the substrate;

2. the coverlayer process, in which the copper patterns are coated with cover layers of electrically insulating material, such as PyraluxOLF Coverlay (manufactured by DuPont Flexible Circuit Materials) ;

3. the mounting of IC-drivers and other components, in which the electrodes of the copper patterns are connected to corresponding control elements;

4. the hole-making process, in which the aperture pattern is produced through the printhead structure, using excimer laser micromachining methods .

5. the cleaning process, in which gas plasma is applied on the substrate for effective removal of all residual material resulting from the hole-making step.

6. the antistatic coating, in which at least one part of the substrate is overlaid with a semi-conductive layer (SCL) for effective removal of residual charges during the print process. According to another embodiment of the invention, the cover layer process is replaced by an insulating process, achieved after the cleaning process, in which the printhead structure, or at least its portion surrounding the apertures, is embedded in a thin film of insulating material such as parylene.

The etching process is achieved in accordance with conventional etching techniques in which a copper surface is cleaned from oxides and laminated with a photo- imaginable resist. The resist is then exposed to collimated UV-radiation, whereby the patterns are developed. The coverlayer process consists in laminating an electrically insulating film, preferably Pyralux® film onto the copper patterns. Pyralux® coverlay composites are constructed of DuPont Kapton® polyimide film, coated on one side with a proprietary B-staged modified acrylic adhesive. Coverlay is used to encapsulate etched details in flexible and rigid-flex multilayer constructions for environmental and electrical insulation. The coverlay is applied onto the copper patterns under heat and vacuum and exposed to collimated UV-radiation via a mask, whereby an appropriate configuration is developed, in which openings are formed in the coverlay on the areas where the IC-drivers and other components are to be mounted. The coverlay is thereafter hardened under heat (about 160°C) and pressure. The next step is to provide the printhead structure with support bars as stiffener for the IC-drivers and other components to be mounted on the substrate. After mounting the components, the apertures are made through the printhead structure. In that step, the printhead structure is fixed on a high- precision X-Y table, and a mask is selected to correspond to the characteristic pitch, i.e. the distance between two adjacent apertures, to define the aperture configuration. The mask is arranged in an optical setting for directing and focusing the laser beams onto predetermined aperture positions. Excimer laser pulses are supplied to the printhead structure at repetition rate up to 100 Hz, whereby the incident energy is absorbed in a thin layer, which is rapidly decomposed, heated and ablated. Each incident laser pulse removes a well defined thin layer of polymer material until the aperture is formed through the substrate and cover layers. As control electrodes have a ring-shaped portion intended to surround a corresponding aperture, that ring- shaped portion can be advantageously used as a mask, so as to ablate the substrate material comprises within the ring-shape portion. In that case the tolerance of the micromachining process can be considerably improved. The apertures are thereafter cleaned in a oxygen based plasma which volatilizes the debris of ablated material that may remain onto the aperture walls. The plasma cleaning is effected in three main steps. In the first step, hydrogen is removed from the carbonaceous debris leaving an activated surface, which, in a second step, further reacts with oxygen to form volatile precursors. Finally, ion bombardment releases the volatile precursors from the wall surface allowing them to be removed from the plasma chamber. In this reaction sequence, the initial extraction of the hydrogen step is usually rate limiting. The oxygen involved in the cleaning reactions are atomic oxygen, and the 0⁺ and 0²⁺ ions. The presence of these species alone however is not sufficient to get useful reaction rates. Ion bombardment is also needed to facilitate the initial attack, and release volatile precursors from the wall surface. This is usually accomplished by introducing small amounts of tertrafluor methane or noble gases to the reaction chamber. Tertrafluormethane breaks down into different species, but primarily CF₃ ⁺ and free fluorine. These fluorinated species facilitate the removal of hydrogen in the initial attack. Adding noble gases, such as helium or argon) to the reaction chamber can also enhance the cleaning process by creating free radicals on the wall surface, and by interacting the oxygen in the chamber to increase the concentration of molecular oxygen to facilitate the desorption of the volatile precursors.

In an alternate embodiment of the invention, the aperture walls and the surface of the printhead structure surrounding the apertures is embedded in a 5-10 microns thin film of insulating material, preferably parylene C, which acts as an insulator and moisture barrier. The printhead structure even includes a semi-conductive layer (SCL) arranged in the surface of the printhead structure facing the transfer belt in the surrounding of the apertures. The SCL provides a well defined resistivity of the surface and is essentially used for drainage of residual charge. The SCL is preferably a 100- 200 πm thin layer of silicium dioxide applied onto the surface using a PVD process (physical vapor deposition) . In some applications, the surface of the printhead structure facing the developer sleeve is brought into frictional contact with the particle layer. In that case, that surface can be overlaid with a wear resistant film having sufficient surface hardness, resistivity, and charge drainage ability. Such a wear resistant film can consist of a diamond-like carbon material, such as amorphous carbon or the like, deposited on the surface in a laser arc method as that described in "Surface and Coating Technology" 85 (1996) 209-214. The laser-arc evaporation process allows an industrial high-rate deposition of amorphous carbon of diamond-like nature. The basic process consists of pulsed cathodic vacuum arc ignited by focused laser pulses.

From the foregoing it will be recognized that numerous variations and modifications may be effected without departing from the scope of the invention as defined in the appended claims.

Claims

What is claimed is;

1. An image forming apparatus in which an image information, formed of a plurality of pixel locations, is converted to a pattern of control signals, which modulate a transport of charged particles from a particle source toward a back electrode member through a printhead structure, whereby the charged particles are deposited onto an image receiving member, interposed between the printhead structure and the back electrode member, and caused to move relative to the printhead structure at a velocity corresponding to one pixel location per print cycle; in which said printhead structure includes : a plurality of apertures for enabling said transport of charged particles such that at least one predetermined pixel location is addressable through each aperture during each print cycle; and control means arranged in conjunction to the apertures for supplying control signals, which, for each pixel location, modulate the transport of charged particles through the corresponding aperture in accordance with the image information, whereby the charged particles transported through said aperture are deposited on the image receiving member at a dot position corresponding to said pixel location, characterized in that a dot position error, defined as the distance by which said dot position diverges from its corresponding pixel location, is less than 50 percents, preferably less than 25 percents of the distance between two neighbouring pixel locations.

2. An image forming apparatus as defined in claim 1, characterized in that said dot position error is less than 20 microns, preferably less than 10 microns .

3. An image forming apparatus as defined in any one of claims 1 - 2 , in which the apertures are arranged at a predetermined gap distance from the image receiving member, characterized in that said dot position error is less than 10 percents of said predetermined gap distance.

4. An image forming apparatus as defined in any one of claims 1 - 3, characterized in that each aperture has a surface conductivity which is sufficiently uniform to provide a dot position error less than 50 percents, preferably less than 25 percents of the distance between two neighbouring pixel locations .

5. An image forming apparatus as defined in claim 1, characterized in that variations of the surface conductivity from one aperture to another are sufficiently small to allow said dot position error to be less than 50 percents, preferably less than 25 percents of the distance between two neighbouring pixel locations.

6. An image forming apparatus as defined in claim 1, characterized in that one pixel location is addressable through each aperture during each print cycle, each print cycle including one print sequence during which the transport of charged particles is controlled along a predetermined trajectory.

7. An image forming apparatus as defined in claim 1, characterized in that several pixel locations are addressable through each aperture during each print cycle, each print cycle including several print sequence during which the transport of charged particles is controlled along several predetermined trajectories, each trajectory corresponding to one of said several addressable pixel locations.

8. A printhead structure for use in a direct electrostatic printing device for modulating a transport of charged particles from a particle source toward a back electrode member, the transported charged particles being deposited at predetermined dot positions on an image receiving member, said printhead structure including: a substrate of electrically insulating material having a predetermined thickness, a first surface facing the particle source and a second surface facing the back electrode member; a first pattern of conductive elements arranged on the substrate, including a plurality of control electrodes ; control voltage sources connected to said control electrodes for selectively controlling the transport of charged particles; and a plurality of apertures arranged through the printhead structure for enabling the transport of charged particles from the particle source toward the back electrode member, each aperture thus having a surface extending in the thickness of the substrate, characterized in that the aperture surfaces are cleaned in a gas plasma treatment during which undesired variations of the surface conductivity of the aperture surfaces are reduced in order to eliminate unpredictable charge interaction between the transported charged particles and the aperture surfaces, thereby reducing dot position error.

9. A printhead structure as defined in claim 8, in which the substrate has a transversal axis extending perpendicular to the direction of a motion of the image receiving member relative to the printhead structure, in which the apertures are aligned in at least one row extending parallel to said transversal axis, in which each aperture corresponds to at least one predetermined addressable dot location along a transversal line on the image receiving member, characterized in that the dot position error, defined as the amount by which the obtained dots diverge from said predetermined addressable dot locations, is less than 50 percents, preferably less than 25 percents of the distance between two neighbouring predetermined dot positions.

10. A printhead structure as defined in any one of claims 8 - 9, in which the apertures are made through the printhead structure by exposing the printhead structure to radiation pulses, whereby incident energy is absorbed in the printhead structure material in a predetermined pattern, said material being thereby heated and ablated, characterized in that residual amounts of said heated and ablated material, contaminating said aperture surfaces , are released from the aperture surfaces during said gas plasma treatment.

11. An image forming apparatus as defined in any one of claims 8 - 10, characterized in that the conductivity of the aperture surfaces is equal to or less than the conductivity of the substrate

12. A printhead structure as defined in any one of claims 8 - 11, further including: a second pattern of conductive elements arranged on said second surface of the substrate, including a plurality of deflection electrodes; deflection voltage sources connected to said deflection electrodes for selectively controlling the transport trajectory of charged particles; a second cover layer of electrically insulating material arranged on said second pattern of conductive elements; characterized in that: each deflection electrode includes a first segment connected to a first deflection voltage source and a second segment connected to a second deflection voltage source, the deflection voltages controlling the transport trajectory of charged particles from the aperture to said predetermined dot positions.

13. A printhead structure as defined in any one of claims 8 - 12, in which said transport trajectory of charged particles is sequentially modified during each print cycle, so as to obtain several addressable predetermined dot positions through each aperture during each print cycle.

14. A printhead structure as defined in any one of claims 8 - 13, characterized in that the aperture surfaces are, after said gas plasma treatment, coated with a layer of electrically insulating material .

15 . A printhead structure as def ined in any one of claims 8 - 14 , further including a layer of semi-conductive material arranged on at least a portion of the printhead structure for removing residual charges from a vicinity of the apertures .

16. A printhead structure as defined in any one of claims 8 - 15, further including a layer of wear resistant material, preferably a layer of diamond-like carbon material, arranged on a portion of the printhead structure intended to be brought into frictional contact with the particle source.

17. A method for producing a printhead structure for use in a direct electrostatic printing device to modulate a transport of charged particles from a particle source toward an image receiving member, said method including the steps of: providing a substrate of electrically insulating material, forming a pattern of conductive elements, including a plurality of control electrodes, on a first surface of said substrate determining a plurality of target positions on said first surface of the substrate, delivering radiation pulses on the target locations for heating and ablating the substrate material to form apertures through the substrate exposing said apertures to a gas plasma treatment to remove residual amounts of said heated and ablated substrate material from the aperture surfaces ; and coating at least a part of the substrate with a coverlayer of electrically insulating material, characterized in that said gas plasma treatment is performed in a reaction chamber containing a gas plasma which is caused to react with the substrate material on the aperture surfaces, so as to volatilize residual amounts of said heated and ablated substrate material .

18. A method as defined in claim 17, characterized in that each of said control electrodes includes a portion, which at least partly surrounds one of said target locations .

19. A method as defined in any one of claim 17 - 18, characterized in that said substrate is made of flexible polymer material, preferably polyimide material .

20. A method as defined in any one of claims 17 - 19, characterized in that said pattern of conductive elements is a copper circuit etched on said first surface of the substrate.

21. A method as defined in any one of claims 17 - 20, characterized in that said substrate has a transversal axis and said target positions are aligned in one row extending parallel to said transversal axis .

22 . A method as defined in any one of claims 17 - 21 , characteri zed in that said subs trate has a transversal axis and said target positions are aligned in at least two parallel rows extending parallel to said transversal axis .

23 . A method as defined in any one of claims 17 - 22 , characterized in that said radiation pulses are generated by a pulsated laser beam delivering an energy, which is absorbed in the substrate material at said target positions .

24. A method as defined in any one of claim 17 - 23, in which each control electrode has a masking portion including an opening, characterized in that said radiation pulses are delivered onto said masking portion of the control electrodes, such that said opening of each masking portion delimits a corresponding target position.

25. A method as defined in any one of claims 17 - 24, characterized in that said radiation pulses are excimer laser pulses generated at a repetition rate up to 100 Hz.

26. A method as defined in any one of claims 17 - 25, characterized in that said gas plasma comprises oxygen, preferably atomic oxygen in the form of 0⁺ and 0²⁺ ions .

27. A method as defined in claim 26, characterized in that said gas plasma further comprises at least one noble gas, such as helium or argon.

28. A method as defined in any one of claims 26 or 27, characterized in that said gas plasma comprises tetrafluormethane (CF₄) .

29. A method as defined in any one of claims 17 - 28, characterized in that said coverlayer is a film of insulating material having a thickness in the range of 10 to 50 microns.

30. A method as defined in any one of claims 17 - 29, characterized in that said coverlayer is a film of insulating material having a dielectric strength in the range of 4000 to 7000 V for 25 microns thickness .

31. A method as defined in any one of claims 17 - 30, further including the step of forming a second pattern of conductive elements, including a plurality of deflection electrodes, on a surface of the substrate opposite to said first surface of the substrate.

32. A method as defined in any one of claim 17 - 31, further including the step of: providing a plurality of control elements arranged on the substrate, and connecting said control elements to the control electrodes, characterized in that each control electrode can be supplied with an electric potential, which, due to control in accordance with an image information, modulates the transport of charged particles through a corresponding aperture.

33. A method as defined in any one of claims 17 - 32, further including the step of : coating at least a portion of the printhead structure with a layer of semi-conductive material.

34. A method for producing a printhead structure for use in a direct electrostatic printing device to modulate a transport of charged particles from a particle source toward a back electrode member, the transported charged particles being deposited at predetermined dot positions on an image receiving member, said method including the steps of: providing a substrate of electrically insulating material having a first surface intended to face the particle source and a second surface intended to face the back electrode member; forming a first pattern of conductive elements on said first surface of the substrate, said first pattern including a plurality of control electrodes; forming a second pattern of conductive elements on said second surface of the substrate, said second pattern including a plurality of deflection electrodes; laminating a first cover layer of electrically insulating material on said first pattern of conductive elements; laminating a second cover layer of electrically insulating material on said second pattern of conductive elements; connecting control voltage sources to said control electrodes and deflection voltage sources to said deflection electrodes; making apertures through the substrate in a predetermined pattern, for enabling the transport of charged particles from the particle source toward the back electrode member, each aperture thus having a surface extending in the thickness of the printhead structure characterized in that the method further includes the step of: exposing the aperture surfaces to a gas plasma treatment during which undesired variations of the surface conductivity of the aperture surfaces are reduced in order to eliminate unpredictable charge interaction between the transported charged particles and the aperture surfaces, thereby improving the dot position control.

35. A manufacturing method as defined in claim 34, in which the apertures are made through the printhead structure by exposing the printhead structure to radiation pulses, whereby incident energy is absorbed in the printhead structure material in a predetermined pattern, said material being thereby melted and ablated, characterized in that amounts of said melted material remaining on the aperture surfaces, are released from the aperture surfaces during said gas plasma treatment .

36. A manufacturing method as defined in any one of claims 34 - 35, characterized in that the distance between two neighbouring apertures is equal to a whole number of the distance between two neighbouring predetermined dot positions.

37. A manufacturing method as defined in any one of claims 34 - 36, characterized in that the apertures are aligned in at least two parallel rows, in which the distance between two neighbouring apertures of a same row is equal to a whole number of the distance between two neighbouring predetermined dot positions .

38. A manufacturing method as defined in any one of claims 34 - 37 , characterized in that the apertures have a substantially circular shape.

39. A manufacturing method as defined in any one of claims 34 - 37 , characterized in that the apertures have a elongated shape having a minor axis extending transversally.

40. A manufacturing method as defined in any one of claims 34 - 38, further including the step of: coating a portion of the printhead structure, which in use is intended to be brought into frictional contact with the particle source, with a layer of wear resistant material, such as diamond like carbon or the like, preferably amorphous carbon applied to said portion using laser arc methods.

AMENDED CLAIMS

[received by the International Bureau on 16 October 2000 (16.10.00); original claims 1 and 9 amended; other claims unchanged (14 pages)]

An image f orming apparatus in which an image inf ormati on , f ormed o f a plural i ty o f pixel locations , is converted to a pattern of control signals , which modulate a transport of charged particles from a particle source toward a back electrode member through a printhead structure , whereby the charged particles are deposited onto an image receiving member , interposed between the printhead structure and the back electrode member , and caused to move relative to the printhead structure at a velocity corresponding to one pixel location per print cycle ; in which said printhead structure includes : a plurality of apertures for enabling said transport of charged particles such that at least one predetermined pixel location is addressable through each aperture during each print cycle ; and control means arranged in conj unction to the apertures for supplying control signals , which , for each pixel location , modulate the transport of charged particles through the corresponding aperture in accordance with the image information, whereby the charged particles transported through said aperture are deposited on the image receiving member at a dot pos i tion corresponding to said pixel location , characteri zed in that a dot pos i tion error , def ined as the distance by which said dot position diverges from its corresponding pixel location, is less than 50 percents, preferably less than 25 percents of the distance between two

An mage form ng apparatus as defined in claim 1 , characterized in that said dot position error is less than 20 microns , pre f erably less than 10 microns .

3. An image forming apparatus as defined in any one of claims 1 - 2, in which the apertures are arranged at a predetermined gap distance from the image receiving member, characterized in that said dot position error is less than 10 percents of said predetermined gap distance.

4. An image forming apparatus as defined in any one of claims 1 - 3, characterized in that each aperture has a surface conductivity which is sufficiently uniform to provide a dot position error less than 50 percents, preferably less than 25 percents of the distance between two neighbouring pixel locations.

An image forming apparatus as defined in claim 1, characterized in that one pixel location is addressable through each aperture during each print cycle, each print cycle including one print sequence during which the transport of charged particles is controlled along a predetermined trajectory.

8. A printhead structure for use in a direct electrostatic printing device for modulating a transport of charged particles from a particle source toward a back electrode member, the transported charged particles being deposited at predetermined dot positions on an image receiving member, said printhead structure including: a substrate of electrically insulating material having a predetermined thickness, a first surface facing the particle source and a second surface facing the back electrode member; a first pattern of conductive elements arranged on the substrate, including a plurality of control electrodes; control voltage sources connected to said control electrodes for selectively controlling the transport of charged particles; and a plurality of apertures arranged through the printhead structure for enabling the transport of charged particles from the particle source toward the back electrode member, each aperture thus having a surface extending in the thickness of the substrate, characterized in that the aperture surfaces are cleaned in a gas plasma treatment during which undesired variations of the surface conductivity of the aperture surfaces are reduced in order to eliminate unpredictable charge interaction between the transported charged particles and the aperture surfaces, thereby reducing dot position error.

9. A printhead structure as defined in claim 8, in which the substrate has a transversal axis extending perpendicular to the direction of a motion of the image receiving member relative to the printhead structure, in which the apertures are aligned in at least one row extending parallel to said transversal axis, in which each aperture corresponds to at least one predetermined addressable dot location along a transversal line on the image receiving member, characterized in that the dot position error, defined as the amount by which the obtained dots diverge from said predetermined addressable dot locations, is less than 50 percents, preferably less than 25 percents of the distance between two

10. A printhead structure as defined in any one of claims 8 - 9, in which the apertures are made through the printhead structure by exposing the printhead structure to radiation pulses, whereby incident energy is absorbed in the printhead structure material in a predetermined pattern, said material being thereby heated and ablated, characterized in that residual amounts of said heated and ablated material, contaminating said aperture surfaces, are released from the aperture surfaces during said gas plasma treatment .

15. A printhead structure as defined in any one of claims 8 - 14, further including a layer of semi-conductive material arranged on at least a portion of the printhead structure for removing residual charges from a vicinity of the apertures .

7 . A method for producing a printhead structure for use in a direct electrostatic printing device to modulate a transport of charged particles from a particle source toward an image receiving member , said method including the steps of : providing a subs trate o f e lec trical ly insulating material , forming a pattern of conductive elements , including a plurality of control electrodes , on a first surface of said substrate determining a plurality of target positions on said first surface of the substrate , del ivering radiation pulses on the target locations for heating and ablating the substrate material to form apertures through the substrate expos ing said apertures to a gas plasma treatment to remove residual amounts of said heated and ablated substrate material from the aperture surfaces ; and coating at least a part of the substrate with a coverlayer of electrically insulating material, characterized in that said gas plasma treatment is performed in a reaction chamber containing a gas plasma which is caused to react with the substrate material on the aperture surfaces, so as to volatilize residual amounts of said heated and ablated substrate material .

18. A method as defined in claim 17, characterized in that each of said control electrodes includes a portion, which at least partly surrounds one of said target locations.

21. A method as defined in any one of claims 17 - 20, characterized in that said substrate has a transversal axis and said target positions are al igned in one row extending parallel to said transversal axis .

26. A method as defined in any one of claims 17 - 25, characterized in that said gas plasma comprises oxygen, preferably atomic oxygen in the form of 0⁺ and 0²⁺ ions

31. A method as defined in any one of claims 17 - 30, further including the step of forming a second pattern of conductive elements, including a plurality of deflection electrodes, on a surface of the substrate opposite to said first surface of the substrate .

33. A method as defined in any one of claims 17 - 32, further including the step of: coating at least a portion of the printhead structure with a layer of semi-conductive material.

34. A method for producing a printhead structure for use in a direct electrostatic printing device to modulate a transport of charged particles from a particle source toward a back electrode member, the transported charged particles being deposited at predetermined dot positions on an image receiving member, said method including the steps of: providing a substrate of electrically insulating material having a first surface intended to face the particle source and a second surface intended to face the back electrode member; forming a first pattern of conductive elements on said first surface of the substrate, said first pattern including a plurality of control electrodes; forming a second pattern of conductive elements on said second surface of the substrate, said second pattern including a plurality of deflection electrodes; laminating a first cover layer of electrically insulating material on said first pattern of conductive elements; laminating a second cover layer of electrically insulating material on said second pattern of conductive elements ; connecting control voltage sources to said control electrodes and deflection voltage sources to said deflection electrodes; making apertures through the substrate in a predetermined pattern, for enabling the transport of charged particles from the particle source toward the back electrode member, each aperture thus having a surface extending in the thickness of the printhead structure characterized in that the method further includes the step of: exposing the aperture surfaces to a gas plasma treatment during which undesired variations of the surface conductivity of the aperture surfaces are reduced in order to eliminate unpredictable charge interaction between the transported charged particles and the aperture surfaces, thereby improving the dot position control.

35. A manufacturing method as defined in claim 34, in which the apertures are made through the printhead structure by exposing the printhead structure to radiation pulses , whereby incident energy is absorbed in the printhead structure material in a predetermined pattern, said material being thereby melted and ablated, characterized in that amounts of said melted material remaining on the aperture surfaces , are released from the aperture surfaces during said gas plasma treatment .

36 . A manufacturing method as def ined in any one of claims 34 - 35 , characterized in that the distance between two neighbouring apertures is equal to a whol e number of the di s tance between two neighbouring predetermined dot positions .