ELECTRODYNAMIC PRINT HEAD WITH SPLIT SHIELDING ELECTRODES FOR LATERAL INK DEFLECTION

Abstract

An electrohydrodynamic print head has a plurality of nozzles arranged in a plurality of wells. Extraction electrodes are located around the wells at a level below the nozzles. Further, shielding electrodes are located around the wells at a level below the extraction electrodes. For each well, there are several such shielding electrodes located at different angular positions. This allows to use the shielding electrodes for laterally deflecting the ink after its ejection from the nozzles.

Claims

1. An electrohydrodynamic print head comprising a plurality of nozzles arranged in a plurality of wells, extraction electrodes located around said wells at a level below said nozzles, shielding electrodes located around said wells at a level below said extraction electrodes, wherein, there are, for each well, several shielding electrodes located at different angular positions adjacent to said well.

2. The print head of claim 1 wherein said shielding electrodes cover at least 90% of a circumference of each well.

3. The print head of claim 1 having several subsets of shielding electrodes, with each subset comprising several electrically interconnected shielding electrodes located at different wells.

4. The print head of claim 3 having at least a first subset-type of shielding electrodes, wherein the shielding electrodes of each set of the first subset-type are connected by interconnect lines located at a vertical level of the shielding electrodes.

5. The print head of claim 4 having at least two subsets of the first subset-type, with a row of said wells being arranged between the shielding electrodes of the two subsets.

6. The print head of claim 3 having at least a second subset-type of shielding electrodes, wherein the shielding electrodes of each set of the second subset-type are connected by vias to interconnect lines located on a vertical level above the shielding electrodes.

7. The print head of claim 5 having at least two subsets of the second subset-type, with a row of said wells being arranged between the shielding electrodes of the two subsets.

8. The print head of claim 1 wherein at least part of said wells have exactly two shielding electrodes located adjacent to said well.

9. The print head of claim 1 comprising a plurality of ventilation openings including blow openings and suction openings.

10. The print head of claim 9 having a regular matrix of nozzles and ventilation openings, wherein, within said matrix, each nozzle is arranged at the center of two suction openings and two blow openings and each ventilation opening is arranged at the center of four nozzles.

11. The print head of claim 9, wherein there is at least a subset A of shielding electrodes and a subset B of shielding electrodes wherein, along a row of nozzles, at a given angular position from said wells, the shielding electrodes of the subset A alternate with the shielding electrodes of the subset B.

12. The print head of claim 1 wherein each shielding electrode covers an angular range of at least 80°around the well.

13. A method for operating the print head of claim 1 for printing on a target, wherein said method comprises applying different electrical potentials to at least some of the shielding electrodes located at different angular positions adjacent to the same well while ink is being ejected from the nozzle in the well.

14. The method of claim 13 comprising mechanically moving said print head with respect to the target below said print head along a direction A, and deflecting ink, using said shielding electrodes, in a direction B, wherein said direction B extends transversally to said direction A.

15. The method of claim 13, wherein the print head has, in a given direction, a spacing S between neighboring nozzles, and wherein for printing a regular structure with a spacing S′along said given direction on the target, wherein said spacing S′ is not equal to or an integer multiple of said spacing S, said method comprises spatially varying, along said given direction, a lateral component of an electrical field generated by said shielding electrodes, to match the spacing of the positions of impact of said ink on said target with the spacing S′.

16. The prnt head of claim 6 wherein the ejection electrodes are located at the same vertical level as said interconnect lines.

17. The print head of claim 1 wherein at least part of said wells have exactly three shielding electrodes located adjacent to said well.

18. The print head of claim 17 wherein one of the three shielding electrodes is a reference electrode extending around an angle of 180°±20° of the well while the other two electrodes are counter-electrodes each extending around angles of 90°±20° of the well.

19. The print head of claim 1 wherein at least part of said wells have exactly tour shielding, electrodes located adjacent to said well.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The invention will be better understood and objects other than those set forth above will become apparent when consideration is given to the following detailed description thereof. Such description makes reference to the annexed drawings, wherein:

[0030] FIG. 1 shows a sectional view of a print head along line I-I of FIG. 2,

[0031] FIG. 2 shows a view along line II-II of FIG. 1,

[0032] FIG. 3 shows a view along line of FIG. 1,

[0033] FIG. 4 shows components of a printer,

[0034] FIG. 5 shows a second embodiment of the shielding electrodes, corresponding to the view of FIG. 2,

[0035] FIG. 6 shows a third embodiment of the shielding electrodes, corresponding to the view of FIG. 2,

[0036] FIG. 7 illustrates a design for compensating alternating ventilation,

[0037] FIG. 8 shows a first application of the deflection technology, and

[0038] FIG. 9 shows a second application of the deflection technology.

MODES FOR CARRYING OUT THE INVENTION

DEFINITIONS

[0039] Terms such as above, below, top, bottom are to be understood such that the nozzle is arranged at a level above the extraction electrodes, and the shielding electrodes are arranged at a level below the extraction electrodes. Advantageously, the axial direction of the nozzles is considered to define the vertical direction.

[0040] Horizontal and lateral designates directions perpendicular to the vertical direction.

[0041] A dielectric is a material having an electrical conductivity of 10.sup.−6 S/m or less.

Print Head Design

[0042] FIGS. 1-4 show a first embodiment of a print head 2 for printing ink on a target 4.

[0043] It comprises a main body 6 with a plurality of structured layers. In particular, main body 6 comprises a nozzle layer 8 and a feed layer 10, with nozzle layer 8 being arranged, by definition, below feed layer 10.

[0044] Nozzle layer 8 farms a plurality of nozzles 12. Each nozzle 12 is arranged in a well 14, namely at a top end of well 14.

[0045] An ejection electrode 16 is provided for each nozzle 12 at a vertical level below nozzle 12. It is structured to electrohydrodynamically extract ink from nozzle 12 and accelerate it towards target 4 below.

[0046] Ejection electrode 16 is advantageously arranged, at least in part, around a well 14 and may in particular be annular, as shown in FIG. 3.

[0047] A plurality of shielding electrodes 18a-18d are arranged at a bottom of nozzle layer 8 at a vertical level below the ejection electrodes 16. These shielding electrodes are used to reduce crosstalk between the nozzles 12, but they are also designed to laterally deflect the ink as it passes the space 22 between print head 2 and target 4. They are described in more detail in the next sections.

[0048] Nozzle layer 8 comprises a plurality of sublayers. In the present embodiment, these include: [0049] A first sublayer 8a forming a bottom section of the wells 14. [0050] A second sublayer 8b located above first sublayer 8a and forming a middle section of the wells 14. [0051] A third sublayer 8c located above second sublayer 8b and forming a top section of the wells 14 as well as the walls of the nozzles 12. [0052] A fourth sublayer 8d arranged above third sublayer 8c and forming a plate carrying the nozzles 12 at the centers of their respective wells 14.

[0053] The sublayers 8a-8d are advantageously dielectric layers, such as layers of inorganic material like silicon dioxide, silicon nitride, silicon oxynitride, or of organic materials like SU8 or BCB (Benzocyclobutene).

[0054] Each nozzle 12 forms a channel 23 extending between a bottomside opening of the nozzle and feed layer 10.

[0055] Nozzle layer 8 may have the same structure at a majority of all nozzles 12 or even at all of them. It may e.g. be mass-produced at a semiconductor foundry using known anisotropic etching and semiconductor patterning technologies.

[0056] Feed layer 10 is e.g. designed as an interposer layer as known from semiconductor manufacturing and it comprises a plurality of ink ducts 24a, 24b extending through it for feeding ink to the nozzles 12.

[0057] In the shown embodiment, the ink ducts comprise via sections 24a, with each via section extending upwards from a nozzle 12 into feed layer 10, where it is connected to an interconnect section 24b. The interconnect sections 24b extend horizontally and interconnect several via sections 24a, and they are in turn connected to one or more ink terminals 26 (FIG. 4) of print head 2, optionally through further vertical via sections and/or horizontal interconnect sections. At the ink terminals 26, the ink ducts are connected to one or more ink reservoirs 28, directly or by means of additional ducts.

[0058] As can be seen from FIG. 3, the ejection electrodes 16 may be connected, by means of electrical tracks, to one or more electrical vias 30, which extend upwards into feed layer 10 (not shown in FIG. 1), where they are suitably wired to ejection electrode terminals 32 (FIG. 4).

[0059] A control unit 34 as shown in FIG. 4 is provided for generating voltage pulses, i.e. voltage pulses between the ejection electrodes 16 and the ink in the nozzles 12, in order to eject ink from the nozzles 12. Advantageously, the voltages of the individual nozzles 12 can be controlled individually or in small groups (with each group containing no more than e.g. 1/100 of all nozzles 12).

[0060] FIG. 1 shows feed layer 10 to comprise a sublayer 10a, which is advantageously a dielectric layer and which forms the via sections 24a of the ink ducts. Feed layer 6 may comprise further sublayers, e.g. the layers 10b-10g of FIG. 1, e.g. for forming further ink duct sections and/or electrical tracks and/or ventilation ducts as described for some of the embodiments below.

[0061] Feed layer 10 can be used for customizing the function of the nozzles 12, e.g. for disabling some of them, e.g. by blocking or interconnecting the ink ducts to some of them and/or the electrical connections to their ejection electrodes 16.

Shielding Electrodes, 1st Embodiment

[0062] The design of the shielding electrodes 18a-18d is best seen in FIGS. 1 and 2.

[0063] In the shown embodiment there are four shielding electrodes 18a-18d located at different angular positions around and below each well 14, with each of them belonging to a different subset of shielding electrodes.

[0064] For each well 14, there is a shielding electrode 18a located at angular position −X from the well, a shielding electrode 18b located at angular position +X from the well, a shielding electrode 18c located at angular position −Y from the well, and a shielding electrode 18d located at angular position +Y from the well.

[0065] The shielding electrodes 18a form a subset of electrically interconnected shielding electrodes. Similarly, the shielding electrodes 18b, 18c, and 18d form their own subsets, with the various subsets being mutually insulated.

[0066] The subset formed by the shielding electrodes 18a is a subset of a “first subset-type”. In a subset of this first subset-type, the shielding electrodes 18a are connected by interconnect lines 40a located at the vertical level of the shielding electrodes 18a that they are connecting, i.e. at the bottom side of first sublayer 8a.

[0067] Similarly, the subset formed by the shielding electrodes 18b is a subset of this first subset-type because they are interconnected by interconnect lines 40b located at the same level as the electrodes 18b.

[0068] The subset formed by the shielding electrodes 18c is a subset of a “second subset-type”. In a set of this second subset-type, the shielding electrodes 18c are connected by means of vias 42a to interconnect lines 44a located on a vertical level above the shielding electrodes 18c (cf. FIG. 3).

[0069] Similarly, the subset formed by the shielding electrodes 18d is a subset of this second subset-type because they are interconnected by means of vias 42b to interconnect lines 44b located on a vertical level above the shielding electrodes 18d.

[0070] As shown in FIG. 3, the interconnect lines 42a, 42b are advantageously located at the vertical level of the ejection electrodes 16, using the space and structured metal layer at this level. This level is e.g. located at the top of first sublayer 8a.

[0071] The assembly of the shielding electrodes 18a-18d into subsets of interconnected electrodes allows to control a plurality of shielding electrodes with the same voltage and simplifies the wiring required in feed layer 10.

[0072] The assembly of the shielding electrodes 18a-18d into subsets of the first and the second subset-type simplifies the horizontal wiring for interconnecting the shielding electrodes of a given subset.

[0073] As can be seen in FIG. 2, a row of wells 14 and nozzles 12 is located between the subsets of the shielding electrodes 18a, 18b. Hence, generating a voltage differential across the electrodes 18a, 1b of these two subsets allows to laterally defleet, along direction X, the ink ejected at all these nozzles in the same manner.

[0074] Similarly, a row of wells 14 and nozzles 12 is located between the subsets of the shielding electrodes 18c, 18d. Hence, generating a voltage differential across the electrodes 18e, 18d of these two subsets allows to laterally deflect, along direction Y, the ink ejected at all these nozzles in the same manner.

[0075] Each subset of shielding electrodes is connected, by means of electrical tracks extending through at least some the layers of the print head, to a deflection terminal, one of which is shown under reference number 46 in FIG. 4. The deflection terminals 46 of the various subsets are connected to control unit 34 for controlling their voltages.

[0076] Similarly, control unit 34 is connected to target 4 or a substrate 48 of target 4, for controlling the electrical field in space 22 between print head 2 and target 4 (cf. FIG. 4).

[0077] In the embodiment of FIG. 2, there are exactly four shielding electrodes 18a-18d located adjacent to each well 14 and nozzle 12.

[0078] In more general terms, at least part of the wells 14 may have exactly four shielding electrodes 18a-18d located adjacent to the well 14.

Shielding Electrodes, 2nd Embodiment

[0079] It is not strictly necessary to have four shielding electrodes 18a 18d adjacent to each well 14 and nozzle 12. In the embodiment of FIG. 5, there are, for each well 14 and nozzle 12, only three shielding electrodes 18a, 18b, 18d.

[0080] Hence, in this embodiment, at least part of the wells 14 have exactly three shielding electrodes 18a, 18b, 18d located adjacent to the well 14.

[0081] When comparing FIG. 5 to FIG. 2, it can be seen that two neighboring shielding electrodes (namely the electrodes 18a, 18c of FIG. 2) have been assembled into a single shielding electrode (namely the electrode 18a of FIG. 2). This embodiment still allows to deflect the ink in direction X (by having a voltage drop across the shielding electrodes 18a, 18b) as well as in direction Y (by having a voltage drop across the shielding electrodes 18a, 18d).

[0082] In the shown embodiment, the shielding electrodes 18a form a subset of the first subset-type and so do the shielding electrodes 18b, i.e. both these subs sets are interconnected by interconnect lines 40a, 40b on the same vertical levels as the shielding electrodes 18a, 18b themselves. On the other hand, the shielding electrodes 18d form a subset of the second subset-type, i.e. they are interconnected by vias 42 connected to interconnect lines (similar to the interconnect lines in 46a of FIG. 3) in a level above the shielding electrodes 18d.

[0083] Advantageously, when there are only three shielding electrodes per well 14 and nozzle 12, one of the shielding electrodes, namely shielding electrode 18a in the shown embodiment, forms a reference electrode and is the largest electrode, while the other two shielding electrodes, namely electrodes 18b and 18d in the shown embodiment, form counter-electrodes and are smaller.

[0084] In particular, the reference electrode extends around 180°±20° of the well 14 and nozzle 12 (see angle α1 of FIG. 5), while the counter-electrodes each extend around 90°±20° of the well 14 and nozzle 12 (see angles α2 and α3 of FIG. 5).

[0085] In this way, the electric field generated between all three electrodes can be regarded as a superposition of a x-deflecting field and a y-deflecting field, originating from the voltage applied between reference electrode 18a and electrode 18b, and from the voltage applied between reference electrode 18a and electrode 18d, respectively. However, it is of course possible to form other electrode shapes, e.g. three electrodes of equal size distributed around the well, advantageously with each electrode extending around 120°±20° of the well 14 and nozzle 12. In this case, however, it may be more difficult to evaluate a certain x-y-deflection value from the voltages applied to the different electrodes.

Shielding Electrodes, 3rd Embodiment

[0086] FIG. 6 shows yet another embodiment with only three shielding electrodes 18a, 18e, 18f located at each well 14 and nozzle 12.

[0087] In contrast to the second embodiment, however, there are two subsets of the second subset-type, with one of these subsets being foamed by the shielding electrodes 18e and the other of these subsets being formed by the shielding electrodes 18f.

[0088] On the other hand, only the shielding electrodes 18a belong to a subset of the first subset-type (even though they may also belong to a subset of the second subset-type).

Ventilation Openings

[0089] The print head 2 may comprise a plurality of ventilation openings 50a, 50b. These include blow openings 50a and suction openings 50b.

[0090] The blow openings 50a are adapted to blow gas into space 22, and the suction openings 50b are adapted to suck gas from space 22, thereby ventilating space 22 for improved ink drying.

[0091] As shown in FIG. 1, the ventilation openings 50a, 50b are connected to ventilation ducts 52a, 52b, 54a, 54b of print head 2, which are in turn connected to a ventilation source 56a and a ventilation sink 56b (cf. FIG. 4).

[0092] Ventilation source 56a is adapted to blow a gas through the ventilation ducts 52a, 54a to the blow openings 50a. Ventilation sink 56b is adapted to suck gas from the suction openings 50b through the ventilation ducts 52b, 54b.

[0093] In one embodiment, all blow openings 50a are connected to the same ventilation source 56a, and all suction openings 50b are connected to the same ventilation sink 56b.

[0094] In a compact embodiment, where at least some of the nozzles 12 and ventilation openings 50a, 50b are arranged in a regular two-dimensional matrix as e.g. shown in FIG. 2, with nozzles extending regularly e.g. along the directions X and Y, respectively, each nozzle 12 is arranged at the center of two blow openings 50a and two suction openings 50b and each ventilation opening 50a, 50b is arranged at the center of four nozzles 12.

[0095] In that case, an alternating flow pattern as illustrated by the arrows 58a, 58b, 60a, 60b in FIG. 7 is created. Namely, the flow pattern will alternate between adjacent nozzles 12.

[0096] Irrespective of the flow direction, the velocity at the nozzle axis becomes zero, which means that the trajectory of droplets that are not actively deflected will not be affected by the alternating flow pattern. However, when deflecting the ink by means of the shielding electrodes, the droplets enter into a non-zero flow field, which can lead to asymmetries in the flight trajectory that may have to be compensated.

[0097] For example, in the embodiment of FIG. 7, let us assume that we want to deflect the ink from all nozzles 12 along direction X by the same amount and not along direction Y. To achieve this, we have to apply a voltage V1 across the nozzles along direction X. If we do so, the droplets ejected from nozzle 12a will experience drag from an air flow corresponding to arrow 60b along direction −Y while the ink from nozzle 12b would experience drag from an oppositely directed air flow corresponding to arrow 60a along direction +Y.

[0098] To compensate for that, alternating auxiliary voltages V2 and −V2 can be applied along direction Y across the wells 14.

[0099] To be able to apply such alternating auxiliary voltages V2 and −V2, there should at least be a subset A of shielding electrodes 18f and a subset B of shielding electrodes 18h. Along a row of nozzles (namely a row extending along direction Y of FIG. 7), when looking at the angular position Y as seen from the wells 14, the shielding electrodes 18f and 18h of the two subsets A and B should alternate.

[0100] In other words, FIG. 7, when looking at the shielding electrodes lying in angular position Y from each well, these shielding electrodes are alternatingly is shielding electrodes 18f and 18h.

[0101] If it is desired to not only deflect the ink into direction X but also into direction Y, the shielding electrodes at the right of the wells 14 of FIG. 7 (i.e. those at angular position +X) should alternate between two subsets A and B as well, as indicated by there being two subsets 18e and 18g alternating with each other along direction X.

Method of Operation

[0102] In order to deflect the inks along the horizontal directions X and/or Y, different electrical potentials can be applied to the shielding electrodes located at different angular positions adjacent to some or all of the wells.

[0103] Typical voltages applied to the various electrodes are e.g. a combination of one or more of the following: [0104] The voltage applied between the ink in the nozzle and the ejection electrode is, for ejection, e.g. in the range of 100V to 500V. [0105] The voltage applied between the ink in the nozzle and the shielding electrodes is typically in the same range as that applied at the ejection electrode, although the voltage may both be higher or lower than that applied at the ejection electrode. [0106] Absolute voltages applied to shielding electrodes on opposite sides of a nozzle are, for maximum deflection, typically between 10V and 100V.

Fast Deflection

[0107] One important application is depicted in FIG. 8. Here, the print head (represented by a single nozzle 12 and its surrounding shielding electrodes 18a-18d) is mechanically moved, in respect to the target, along a horizontal direction A while ejecting ink.

[0108] At the same time, the ink is deflected by means of the shielding electrodes in a direction B, which is perpendicular (or transversal) to direction A.

[0109] Hence, it becomes possible to print at positions that are not directly below nozzle 12.

[0110] Advantageously, the lateral displacement velocity of the ink position on the target in direction B by means of the electrostatic deflection is faster than the lateral displacement of the ink position on the target in direction A by means of mechanical displacement, in particular at least 10 times faster. This allows to generate a high resolution print along both directions without fast mechanical displacements.

[0111] This technique allows to move print head 2 without acceleration (or without large acceleration) along A while the point of impact oscillates along direction B.

[0112] If print head 2 moves steadily along direction A and it is desired to generate series of dots exactly along direction B, i.e. a direction exactly perpendicular to A, as shown in FIG. 7, the shielding electrodes arranged across the nozzle along direction B (i.e. the electrodes 18a, 18b in the shown example) can be used for the lateral deflection along direction B while the shielding electrodes arranged across the nozzle along direction A (i.e. the electrodes 18c and 18d in the shown example) can be used to compensate for the continuous forward movement of print head 2 along direction A.

[0113] Advantageously, the voltages along directions A and B would be sawtooth-shapes voltages, i.e. each of them changes from a first voltage to a second voltage, in particular continuously, during a first time interval T1, and then goes back to the first voltage in a second time interval T2, with T1>>T2, in particular T1>10.Math.T2.

[0114] It must be noted that, in order to implement the technique of FIG. 8, a print head with only three shielding electrodes, such as the one of FIG. 6, can be used as well.

Alignment Correction

[0115] In certain situations it can be beneficial that not all nozzles on the print head are individually controllable, but instead the ejection electrodes 16 of some nozzles may be interconnected and are therefore ejecting droplets always at the same time. Print heads with such characteristics can be used if a regular structure 64 is to be printed. In this case, the interconnected nozzles 12 on the print head may be arranged in reference to a regular structure 64 that needs to be printed on. When initialing printing, the number of interconnected nozzles 12 will define the number of regular structures 64 that is printed on at the same time. However, when doing so, one implies that the reference spacing S between neighboring nozzles 12 is exactly the same as the spacings S′ defining the regular structure 64. Due to various reasons, these distances may be different though, so another application of deflection by means of the shielding electrodes is depicted in FIG. 9, where the deflection is used to correct for registration mismatches between the nozzles 12 and the regular structures 64.

[0116] For example, print head 2 is supposed to print onto a regular structure 64 contained on substrate 4 with a spacing S′ along direction D while it is moving in a horizontal forward direction perpendicular to D, i.e. in a direction perpendicular to the plane of FIG. 9. Spacing S′ of the structure 64 is, however, not a multiple integer of spacing S, i.e. in a conventional print head it would be necessary to displace the print head laterally not only along its forward direction but also along direction D in order to print accurately on all structures 64, which would not only require additional mechanical movement but also reduce the printing speed substantially.

[0117] However, if the shielding electrodes are used to laterally deflect the ink (i.e. along direction D), this can be achieved without laterally displacing print head 2 along direction D.

[0118] In order to print structure 64, the component of the electric field along direction D is statically varied along direction D in order to match the spacing of the positions of impact of the ink on target 4 with the spacing S′.

[0119] In the example of FIG. 9, spacing S′ is somewhat larger than spacing S. Hence, the ink needs to be spread along D by deflecting the ink from the leftmost nozzles 12 slightly to the left and from the rightmost nozzles 12 slightly to the right.

[0120] This is particularly important when the print head has a large extension along direction D. In that case, different temperatures at print head 2 and target 4 combined with different thermal dilatations of print head 2 and target 4 may affect the spacings S and S′ differently. Hence, even if at one set of temperatures, the spacing S and S′ were matched perfectly, a change of temperature would lead to a mismatch.

[0121] For example, the centermost nozzles 12 of print head 2 may be well-aligned over the structure 64. In that case, ink of the outermost nozzles 12 will need a lateral correction.

[0122] Hence, along direction D, there are advantageously several different subsets of shielding electrodes, which allows to apply a different voltage differential over the nozzles at the center and those further away (along direction D) from the center, thereby adapting the deflection along direction D.

[0123] In some cases, it may e.g. be sufficient to use the same voltage differential over e.g. all electrodes within a region of 10 mm. If the printing head has an to extension, along D, of e.g. 30 mm, three regions of different subsets may in that case suffice.

[0124] The correction depicted in FIG. 9 can also be used in both horizontal directions, i.e. also along the horizontal direction perpendicular to direction D.

Notes

[0125] In the embodiments above, there is at least one subset of shielding electrodes of the first subset-type, i.e. they are connected by interconnect lines located on the same vertical level as the shielding electrodes themselves. Alternatively, though, there may only be subsets of shielding electrodes of the second subset-type, i.e. there are no interconnect lines 40a, 40b on the level of the shielding electrodes 18a-18f. Rather, all shielding electrodes 18a-18f are connected to vias (such as the vias 42a, 42b) and to interconnect lines (such as lines 44a, 44b of FIG. 3) on a vertical level above the shielding electrodes 18a-18f. This allows to generate a shielding electrode pattern of higher symmetry.

[0126] In the embodiments above, there are three or four shielding electrodes at each nozzle 12 and well 14. If deflection only along one direction is desired (such as direction D of the application of FIG. 9) and no venting compensation of the type illustrated in FIG. 7 is desired, it may be sufficient to have only two shielding electrodes located adjacent to said well.

[0127] As already mentioned, the shielding electrodes should cover a large percentage of the area around each well 14, e.g. at least 90% of its circumference, in order to shield the field of the ejection electrode 16 and prevent crosstalk between neighboring nozzles 12.

[0128] As mentioned above, the shielding electrodes of a given subset can be interconnected at the vertical level of the electrodes or at the vertical level of the ejection electrodes. However, in particular if the subsets have a more complex geometry, such as the one shown in FIG. 7, a further interconnection layer can be introduced, e.g. by splitting first sublayer 8a into two sub-sublayers and arranging at least some of the interconnect lines between the two sub-sublayers (with vias connecting them to the shielding electrodes).

[0129] While there are shown and described presently preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practiced within the scope of the following claims.