WO2006059095A1

WO2006059095A1 - Droplet deposition

Info

Publication number: WO2006059095A1
Application number: PCT/GB2005/004587
Authority: WO
Inventors: Paul Raymond Drury; Robert Alan Harvey
Original assignee: Xaar Technology Limited
Priority date: 2004-11-30
Filing date: 2005-11-30
Publication date: 2006-06-08
Also published as: GB0426221D0; KR20070091170A; JP2008526468A; CN101069297A

Abstract

A method for depositing fluid from an array of ejection nozzles (206) into physical regions (202) defined in a substrate (204) by effecting relative movement of a printhead (208) and the substrate such that each of said plurality of target regions (202) is addressed by more than one nozzle and ejecting droplets (212) from a plurality of nozzles to form a fluid envelope in each region (202). The positioning perpendicular to the direction of movement (210) of the fluid envelope within said region is controlled by the proportion of fluid ejected from each of said more than one nozzles (206). The positioning of the fluid envelope in an orthogonal direction can be controlled by ejection timings.

Description

DROPLET DEPOSITION

The present invention relates to deposition of fluid onto a substrate having complementary features for receiving fluid, and particularly but not exclusively to depositing fluid into an array of wells for the manufacture of displays.

The manufacture of displays such as Organic Light Emitting Diode (OLED) displays typically involves depositing a discreet volume of fluid between electrode sheets to form a single element of the display. A large array of such elements can be achieved by depositing fluid in a substrate formed with an array of 'wells' defined by raised banks. The geometry of the wells will tend to vary between applications and with the desired resolution of the display being manufactured. A screen for a handheld device may require a relatively small well size, while in a wall mounted display, the well size will tend to be much larger.

It is important that each well is filled with the correct volume of fluid. If a well receives too little fluid, the resulting pixel of the display will tend to appear brighter than desired. It is also important for the wells to be filled evenly, since if the fluid layer is too thin at any point, the electrode sheets can short circuit causing failure of that element.

The wells used in the manufacture of such displays are typically arranged in groups of three, corresponding to a red, green and blue element side by side. Such an array of wells is shown in Figure 1. It can be seen that each well is substantially rectangular, a group of three wells being approximately square.

Figure 1 shows a substrate having an array of substantially rectangular wells 102, separated by banks 104. It has previously been proposed to fill the wells with fluid using droplet deposition by arranging for the nozzles 106 of a printhead 108 to be aligned with the shorter dimension of each well. This typically involves arranging the printhead at a steep angle as shown. In this way each well is filled by a single nozzle firing a series of drops as shown at It has been found by the present inventors that this method of depositing fluid in the wells can result in uneven distribution of fluid in each well, and uneven distribution of fluid between the wells

According to a first aspect, the present invention provides a method for ejecting fluid droplets from an array of ejection nozzles extending in an array direction onto a substrate having a plurality of physically defined target regions for accepting fluid, the method comprising the steps of effecting relative movement of the printhead and the substrate in a first direction such that each of said plurality of target regions is addressed by more than one nozzle; ejecting droplets from a plurality of nozzles such that each of said plurality of target regions receives droplets from more than one nozzle, said droplets having a fluid envelope on the substrate; wherein, for each of said target regions, the positioning perpendicular to said first direction of the fluid envelope within said physically defined target region is controlled by the proportion of fluid ejected from each of said more than one nozzles.

It has been found that, although alignment between the nozzle spacings and the target regions may not be as accurate as in the method described with respect to Figure 1 , a number of advantages are afforded by this novel method. By depositing fluid in each target region from a plurality of nozzles, the region may be filled substantially simultaneously, as opposed to being filled by a single nozzle depositing fluid in successive droplets. It has been found that this promotes more even filling of the target regions.

Furthermore, in the method of Figure 1 , an entire column of target regions is filled by a single nozzle. Should this nozzle be deficient in any way, an entire column of pixels of a display made in this way will exhibit a deficiency, which will produce a noticeable fault in the display. By using the present method, any deficiency between nozzles will tend to be averaged out within a single target region, and between target regions. It is important to appreciate that the target regions will not typically be at a spacing native to the printhead, in either the direction of relative movement or perpendicular thereto. It is also important to appreciate that it is not sufficient for fluid envelopes (which is to say the footprint of a volume of fluid on the substrate) to be aligned relative to one another, and that each envelope must be aligned with its intended target region. The problem, addressed by the present invention, of achieving such alignment whilst maintaining the advantages of traversing each target region with multiple nozzles is therefore non-trivial.

Further embodiments of the invention are set out in the dependent claims

The present invention will now be described, by way of example only, with respect to the accompanying drawings in which:

Figure 1 illustrate a prior art method of depositing fluid in an array of wells

Figure 2 shows a method of depositing fluid in an array of wells according to the present invention.

Figure 3 shows a fluid envelope formed from a plurality of droplets in a well of a substrate.

Figures 4a and 4b illustrate a well with fluid pinned to the edges of the well

Figures 5 and 6 illustrate an ejection timing packet, and variation of droplets within such a packet.

Figure 7 illustrates a fluid envelope formed from an array of droplets extending in two directions

Referring to Figure 2, a substrate having an array of wells 202 separated by banks 204 are shown. Here the wells have curved edges resulting in an elongate oval shape. The wells might typically be 10Omicrometers in length and 25 micrometers in width, for a screen resolution of 200 pixels per inch. In general though, well dimensions will tend to vary in both shape and size. An array of nozzles 206 of a printhead 208 is aligned with the long axis of the wells and perpendicular to the short axis of the wells to traverse the substrate in the direction indicated by arrow 210. Each well is traversed by a plurality of nozzles which can be used to eject fluid into the wells. Nozzles which are aligned with a bank running parallel to the direction of movement are not used. A plurality of drops 212 can therefore be ejected, from different nozzles but substantially simultaneously to fill a well with fluid. It will be understood that the droplets ejected into a well will spread and merge to form a fluid envelδpe^'δrfthe subsfraterThe^~sprea^"ding^~andmerging-will be largely determined by the surface properties of the substrate and the fluid used.

Figure 3 illustrates a fluid envelope formed by a series of droplets ejected from a group of adjacent nozzles. It can be seen that by varying the proportions of fluid ejected by each nozzle, the shape of the envelope can be varied to complement the dimensions of the well, and can have less than two axes of symmetry. Here drop 302 at the end of the group defines the uppermost edge of the envelope. Because of potential misalignment of the nozzles and the wells, using a standard drop size will result in the fluid envelope not reaching completely to the bank 308, and a further droplet added to the edge of the group would impinge on the bank. Droplet 302 has therefore been made larger than the rest of the droplets from other nozzles in order to better fill the well. Drop 304 at the other end of the group has also been made larger, but not as large as drop 302 so as to effectively fill the well but without depositing fluid on a bank 306. The remainder of the drops contributing to the fluid envelope can be chosen to evenly fill the space between the two edge drops and can be selected to ensure a desired total volume of fluid is deposited.

In conventional printing the drop size is typically adjusted to form a dot on the page of diameter 2Vs, where s is the nozzle spacing of the printhead. It will be understood that this can achieve full coverage by ensuring that each dot touches a diagonal neighbour in a square array of dots. So-called 'greyscale' printing allows smaller dot sizes to be used to generate a wider range of print tones, and is discussed in more detail below. Here however dot sizes larger than 2Vs can advantageously be used to achieve a desired fluid placement. Advantages of traversing the substrate with a printhead in the manner shown in Figure 2 have been discussed above, however it will be appreciated that since an array of wells may have arbitrary spacings, it cannot be ensured that nozzles will be regularly arranged with respect to wells in the direction perpendicular to movement of the printhead. Considering the exemplary well dimen^"siόns^"descTibiai^tlTTe^pecf to Figure 27and taking a nozzle spacing of 1440 dpi, it will be understood that each well will be addressed by approximately 6 nozzles, however depending on the geometry of the substrate some wells may be addressed by more nozzles that others.

By varying the size of droplets as shown in Figure 3 though, it can be seen that accurate registration of nozzles with respect to the edges of wells lying in the direction of movement (ie edges illustrated as running horizontally in Figure 3) is not necessary to ensure accurate registration of the fluid envelope deposited with these edges.

As noted above, spreading and merging of the fluid in the substrate depends on the surface properties of the substrate and the fluid deposited. In certain applications, the substrate will be non-porous, and it has been found that accurate fluid deposition with respect to the edges of the wells is critical in filling the well uniformly. Figures 4a and 4b illustrate a body of fluid 402 deposited in a shaped substrate 404. It can be seen that the volume of fluid deposited is greater than the volume of the well, and is held in place over the well by surface tension. By depositing fluid in this way the side walls 406 of the wells are wetted by the fluid, and in certain applications it may even be desirable to wet a portion of the top of the bank 408 with the deposited fluid. By wetting at the edges of the well, the envelope of fluid in the well is effectively 'pinned' to those edges and is prevented from receding away from the edge.

In the example of Figure 4, excess fluid is then allowed to evaporate from the well, leaving an even layer of fluid 412 in the well. Because the edges of the fluid envelope have been 'pinned' to the edges of the well, the remaining fluid extends evenly into the extremities of the well. Even in applications which do not require the well to be filled initially with excess fluid, pinning of fluid to the well edges is important in ensuring even distribution of fluid within the well.

While the positioning of the fluid envelope perpendicular to the direction of movement relative to the substrate has been discussed, the fluid envelope should equally be cόTTtrόlleciirrtrTe^'ortrrogonal^'direction. Whilst in the method described with respect to figure 1 this is achieved by specific alignement of the printhead, in the present invention, as already stated, dedicated alignment of the printhead to the well array is not always possible. A method of control of such positioning involving ejection timings is discussed below.

A preferred method for varying the droplet size as discussed above involves generating from a single nozzle a series of sub-droplets of substantially fixed volume which combine to form a droplet, the volume of which depends on the number of constituent sub-droplets. The subdroplets may merge at the nozzle plate, in flight, or on the substrate. Preferred techniques for greyscale printing are described in WO 96/10488.

Figure 5 illustrates a 'packet' 502 of fifteen subdroplets capable of sixteen levels of grey. The packet can be thought of as a data stream for the ejection timing of sub-droplets, or as a string of sub-droplets in flight, assuming that no merging takes place in flight and that velocities are equal for all subdrops.

It will be understood that each of the possible timings in the packets is addressable, and different droplet sizes can be achieved by controlling the number of subdroplets ejected. Packet 504 for example illustrates ejection of three of the possible fifteen subdroplets to form a relatively small droplet. Packet 506 on the other hand illustrates ejection of all fifteen possible subdroplets to create the maximum droplet size.

Certain embodiments of the present invention take advantage of the packet structure used in such greyscale printing to vary the placement of a droplet in the direction of relative movement with the substrate in fine increments. Figure 6 shows a packet of fifteen possible subdroplets. At 602, a droplet of three subdroplets is shown being ejected at the 'centre' of the packet illustrated by line 608. The droplet formed 612 will be placed on the substrate on a nominal centre line 610. For a droplet of the same size, it is possible to vary the placement on the substrate by varying the position of the subdroplets within the packet. At 604, a droplet having advanced flight is illustrated, and the corresponding placement δn the substrate is^"shown^~at 61 ^displaced from centre line 610 by an amount in the substrate scanning direction indicated by arrow 620. Droplet 616, displaced by an equal but opposite amount from, nominal line 610 is formed by delaying the flight using the packet structure shown at 606.

By arranging for the printhead drive electronics to be able to address a packet having a size greater than the maximum droplet size, fine adjustment of the ejection timing and hence placement position in this way is possible for all droplet sizes. It will be understood that droplets smaller than the maximum size will have a greater range of placement variation.

It can be seen therefore that control of the fluid envelope in the direction of relative movement of the fluid can be finely adjusted by controlling ejection timings in this way. In the case of an arrangement such as that of Figure 3, the ejection timing can be fine tuned to align the fluid envelope with the longitudinal centre of the well.

Figure 7 depicts an embodiment in which the fluid envelope used to fill a well is made up of a series of droplets in both the direction of relative movement of the substrate and the direction perpendicular thereto. In this case the control over placement of the envelope by ejection timing described above can be used to align edges of the fluid envelope to edges of banks 702 and 704. In other words, a 'corner' droplet such as 706 is controlled in the direction of relative movement by ejection timing to be aligned to bank 704, and in a perpendicular direction by varying the droplet volume to be aligned to bank 708. In this way, the fluid envelope can be 'pinned' to the edge of the banks in all directions. Although a substrate having an array of wells has been described, the invention is equally applicable to a substrate having target regions physically defined by surface properties of the substrate. For example, target regions may comprise hydrophilic portions of a substrate separated by hydrophobic portions

Claims

1. A method for ejecting fluid droplets from an array of ejection nozzles extending in an array direction onto a substrate having a plurality of physically defined target regions for accepting fluid, the method comprising the steps of: effecting relative movement of the printhead and the substrate in a first direction such that each of said plurality of target regions is addressed by more than one nozzle; ejecting droplets from a plurality of nozzles such that each of said plurality of target regions receives droplets from more than one nozzle, said droplets having a fluid envelope on the substrate; wherein, for each of said target regions, the positioning perpendicular to said first direction of the fluid envelope within said physically defined target region is controlled by the proportion of fluid ejected from each of said more than one nozzles.

2. A method according to Claim 1 , wherein the proportions of fluid delivered to each target region from said more than one nozzles is different.

3. A method according to Claim 1 or Claim 2, wherein at least one edge of the fluid envelope in said first direction is aligned with at least one physical edge of said target region by controlling the droplet sizes ejected from said more than one nozzles.

4. A method according to any preceding claim, wherein the droplet size ejected from at least one of said more than one nozzles is greater than 2Vs, where s is the nozzle spacing of the printhead.

5. A method according to any preceding claim, wherein each target region receives fluid from a group of adjacent nozzles, and droplets from nozzles at the edge of said group define the edge of said fluid envelope perpendicular to said first direction, wherein the droplet size of at least one nozzle at the edge of said group is controlled to pin said fluid envelope to at least one physical edge of said target region.

6. A method according to any preceding claim, wherein each target region receives fluid from a group of adjacent nozzles, and wherein droplets from nozzles at the edge of said group are larger than droplets in the centre of said group.

7. A method according to any preceding claim, wherein the size of droplets ejected from said more than one nozzle into each target are is controlled so as to deliver a predetermined total volume of fluid to each target area.

8. A method according to any preceding claim, wherein the plurality of droplets forming said fluid envelope are deposited substantially simultaneously.

9. A method according to any preceding claim, wherein placement of fluid in the first direction is determined by the ejection timing.

10. A method according to Claim 9, wherein for each of said target regions the positioning in said first direction of the fluid envelope within said physically defined target region is separately controlled by the ejection timing of said more than one nozzles addressing each said target region.

11. A method according to Claim 9 or Claim 10, wherein the fluid envelope is controlled to be aligned with the centre of said physically defined target region in said first direction.

12. A method according to any one of Claims 9 to 11 , wherein an edge of the fluid envelope perpendicular to said first direction is controlled to be aligned with at least one physical edge of said target region.

13. A method according to any one of Claims 9 to 12, wherein droplets making up said fluid envelope are formed by a series of subdroplets ejected from an ejection timing packet containing a plurality of possible subdroplet ejection timings, and wherein adjusting the ejection timing of a droplet comprises varying the position of the series of subdroplets within said packet.

14. A method according to Claim 13, wherein the ejection packets of said more than one nozzles are co-timed.

15. A method according to Claim 13 or Claim 14, wherein a series of ejection packets are repeated at a droplet ejection frequency.

16. A method according to any one of Claims 13 to 15, wherein said timings are equally spaced apart in said packet.

17. A method according to any preceding claim, wherein said substrate comprises a substantially regular array of target regions.

18. A method according to any preceding claim wherein the largest dimension of said target regions extends substantially perpendicular to said direction of relative movement.

19. A method according to any preceding claim wherein said target regions are substantially rectangular.

20. A method according to any preceding claim wherein said substrate comprises an array of wells separated by banks.

21. A method according to any preceding claim wherein said first direction is substantially perpendicular to said array direction.