Techniques for Print Ink Droplet Measurement and Control to Deposit Fluids within Precise Tolerances

#This disclosure relates to techniques for measuring ink jet droplet volumes used for organic light-emitting diode (“OLED”) device fabrication with a high degree of statistical accuracy, to use of a printing process to transfer droplets of a fluid ink to target regions of a substrate in precise aggregate quantities, and to related methods, devices, improvements and systems. In one non-limiting application, techniques provided by this disclosure can be applied to a manufacturing process for OLED display panels.

#In a printing process where a printhead has multiple nozzles, not every nozzle reacts to a standard drive waveform the same way, i.e., each nozzle can produce a droplet of slightly different volume. In situations where the nozzles are relied upon to deposit fluid droplets into respective fluid deposition areas (“target regions”), lack of consistency can lead to problems. This is particularly the case for manufacturing applications, where the ink transports a material that will become a permanent thin-film structure within an electronic device. One example application where this issue arises is in a manufacturing process applied to the fabrication of displays, such as organic light-emitting diode (“OLED”) displays, as used for small and large electronic devices (e.g., for portable devices, large scale high-definition television panels and other devices). Where a printing process is used to deposit an ink carrying light-generating materials of such displays, the volume discrepancy across rows or columns of pixels contributes to visible lighting or color defects in a displayed image. Note that “ink” as used herein refers to any fluid applied to a substrate by nozzles of a printhead irrespective of color characteristics; for example, in the mentioned OLED display fabrication application, ink is typically deposited in place and then processed, dried or cured in order to directly form a permanent material layer, and this process might be repeated with the same ink or a different ink to form several such layers.

#FIG. 1A is used to introduce this nozzle-droplet inconsistency issue, with an illustrative diagram generally referenced using numeral 101. In FIG. 1A, a printhead 103 is seen to have five ink nozzles, which are each depicted using small triangles at the bottom of the printhead, each respectively numbered (1)-(5). Note that in a typical manufacturing application, there can be many more than five nozzles, e.g., 24-10,000, depending on application; in the case of FIG. 1A, five nozzles are referenced simply for ease of understanding. It should be assumed that in an example application, it is desired to deposit fifty picoliters (50.00 pL) of a fluid into each of five specific target regions of an array of such regions, and further, that each of five nozzles of a printhead is supposed to eject ten picoliters (10.00 pL) of fluid with each relative movement (“pass” or “scan”) between the printhead and a substrate into each of the various target regions. The target regions can be any surface areas of the substrate, including adjoining unseparated areas (e.g., such that deposited fluid ink partially spreads to blend together between regions), or respective, fluidically-isolated regions. These regions are generally represented in FIG. 1A using ovals 104-108, respectively. Thus, it might be assumed that exactly five passes of the printhead are necessary as depicted to fill each of the five specific target regions. However, printhead nozzles will in practice have some minor variations in structure or actuation, such that a given drive waveform applied to respective nozzle transducers yields slightly different droplet volumes for each nozzle. As depicted in FIG. 1A, for example, the firing of nozzle (1) yields a droplet volume of 9.80 picoliters (pL) with each pass, with five 9.80 pL droplets being depicted within oval 104. Note that each of the droplets is represented in the figure by a distinct location within the target region 104, but in practice, the location of each of the droplets may be the same or may overlap. Nozzles (2)-(5), by contrast, yield slightly different, respective droplet volumes of 10.01 pL, 9.89 pL, 9.96 pL and 10.03 pL. With five passes between printhead and substrate where each nozzle deposits fluid on a mutually-exclusive basis into the target regions 104-108, this deposition would result in a total deposited ink volume variation of 1.15 pL across the five target regions; this can be unacceptable for many applications. For example, in some applications, discrepancy of as little as one percent (or even much less) in deposited fluid can cause issues; in the case of OLED display fabrication, such variation can potentially result in image artifacts observable in a finished display.

#FIG. 1A is a diagram that presents a hypothetical problem of depositing ink in target regions of a substrate where a printhead with five nozzles is used to deposit a target fill of 50.00 pL in each of five specific target regions.

#FIG. 1B is another diagram that presents a hypothetical problem of depositing ink in target regions of a substrate where a printhead with five nozzles is used to deposit a target fill of 50.00 pL in each of two specific target regions.

#FIG. 2A is an illustrative diagram that shows a droplet measurement system capable of measuring droplet volumes for each nozzle of a large printhead assembly.

#The subject matter defined by the enumerated claims may be better understood by referring to the following detailed description, which should be read in conjunction with the accompanying drawings. This description of one or more particular embodiments, set out below to enable one to build and use various implementations of the technology set forth by the claims, is not intended to limit the enumerated claims, but to exemplify their application. Without limiting the foregoing, this disclosure provides several different examples of techniques used to fabricate a materials layer by planning printhead movement so as to maintain deposited ink volume within predetermined allowances while not excessively increasing the number of printhead passes (and thus the time needed to complete a deposited layer). In connection with these techniques, accurate droplet measurement can be performed so as to accurately plan composite ink fills in any target region, with measurement highly integrated with production printing. The various techniques can be embodied as software for performing these techniques, in the form of a computer, printer or other device running such software, in the form of control data (e.g., a print image) for forming a materials layer, as a deposition mechanism, or in the form of an electronic or other device (e.g., a flat panel device or other consumer end product) fabricated as a result of these techniques. While specific examples are presented, the principles described herein may also be applied to other methods, devices and systems as well.

#This disclosure relates to use of a printing process to transfer layer material to a substrate, techniques for droplet measurement with a high degree of accuracy, and related methods, improvements, devices and systems.

#The nozzle consistency issue introduced above can be addressed by measuring droplet volume per nozzle (or variation in droplet volume across nozzles) of a printhead for a given nozzle firing waveform. This permits planning of printhead firing patterns and/or motion to deposit precise aggregate fill volumes of ink in each target region. With an understanding of how droplet volume varies across nozzles, printhead/substrate positional offsets and/or droplet firing patterns can be planned in a manner that accommodates differences in droplet volumes but that still optimizes concurrent printing in adjacent target regions with each pass or scan. Viewed from a different perspective, rather than normalizing or averaging out nozzle-to-nozzle variation in droplet volumes, the specific droplet volume characteristics of each nozzle are measured and used in a planned manner to concurrently achieve specific in-range aggregate volumes for each of multiple target regions of the substrate; in many embodiments, this planning is performed using a process that reduces the number of scans or printhead passes in dependence on one or more optimization criteria.