Development of a Femtoliter Piezo Ink-jet Head for high resolu- tion printing

Inkjet printing technology is used in the manufacture of color Þlters and alignment layer Þlm for liquid crystal displays. The minimum droplet volume is achieved under a few picoliters and the head prints lines of a few tens of micrometers. In order to expand range of applications of the inkjet technology, we consider that a inkjet head is needed with the ability to micropattern feature sizes of 10 micrometers. The size of features depends mainly on the volume of ink contained in droplets Þred from an inkjet head. This work presents a new inkjet head comprising piezoelectric actuators that Þres droplets having a volume of less than 1 picoliter. Examples in which the inkjet head was used to directly deposit ink droplets to micropattern devices are described. An efÞcient approach has been taken to develop the inkjet head so that the head structures are optimized and micro-droplet behavior is controlled. The inkjet head is found to be capable of Þring 500femtoliter droplets and forming micropatterns on a substrate. The droplets exhibit a stable, straight ßight path. Intoduction Inkjet printing is a familiar method for transferring electronic data to paper and for forming photo-like prints, and inkjet printers are standard equipment in every ofÞce and household. On the other hand, Inkjet printing has been expanding its application area where a variety of functional materials such as polymers and metals are deposited in the form of ink containing these materials to fabricate electronic devices. Today, inkjet fabricated multicolor polymer light-emitting diodes, metal lines and liquid crystal displays are well known examples of the new application area of the inkjet technology [1, 2]. Additionally, the scope of application is widely believed to have untapped market. One of the major challenges in inkjet printing is to develop a system capable of Þring the very small droplets needed to draw narrower lines on substrates. Seiko Epson has been focusing on the development of piezoelectric inkjet technology that produces high-resolution, high-quality prints on paper using a Multi-layer Actuator Head (MACH) that is driven with a piezoelectric element. The ejection of droplets as small as 1.5 picoliters has enabled extremely high dot density marking with reduced dot-to-dot spacing. In addition, Variable Sized Droplet Technology (VSDT) [3], which enables the discharge of three different sizes of droplets (small, medium and large) from the same nozzle by changing a driving waveform, has evolved to the extent that the different sized droplets are selectively discharged according to input image data, thus boosting printing speed. Many researchers use an inkjet process to deposit nano-metal particles to produce circuit boards and ßat-panel displays. Most of these processes use common inkjet heads that can Þre droplets having a volume on the order of several picoliters. As a result, the widths of drawn lines the head can draw are in the range of a few tens of micrometers to hundredths of micrometers. Screenprinting can also draw linewidths on this same order. On the other hand, photolithography techniques can be used to form linewidths measuring several micrometers or less. The disadvantage with photolithography is that expensive photo-masks must be prepared for each pattern, thus adding to device cost. In the printer business, dots created using droplets having a volume of 1 picoliter or less are not considered intense demands, as the human eye is incapable of resolving dots this small. In device manufacturing, however, droplets of less than 1 picoliter have great value. A 1-picoliter droplet has an estimated diameter of 15 micrometers or less after landing on a substrate. Taking wetting on substrates into consideration, the width of a line drawn with 1-picoliter droplets grows to 30 micrometers or more (Figure 2). Inkjet-drawn linewidths of 10 micrometers or less are considered to be the threshold beyond which applications will expand. The purpose of our work was to develop a new inkjet head capable of discharging a femtoliter-order droplet. In addition, to characterise the developed print heads, performance and quality of drawn lines were examined. Figure 1. The ßight image of 4000-femtoliter droplets(upper) and 500femtoliter droplets(lower). 898 Society for Imaging Science and Technology Head design Drop size modulation demands consideration of many parameters, such as hydrodynamics, head structure and driving waveform. Therefore, we Þrst performed a ßow simulation. The simulation was performed with FDM to clarify whether or not femtoliter-order droplets could be Þred [4, 5]. We indicate the intention to reduce the volume. Small volume makes the ßight speed V decrease. Therefore, characteristic vibration period of a pressure chamber Tc needs to be reduced , or a cross section area of nozzle A to be smaller. If ink volume Q is reduced to a third part of a current volume, the product of Tc and A also have to be a third part the current one to keep the ßight speed unchanged. We reduced a nozzle diameter and as a result, A became smaller. We made several type print heads , which have different size of nozzle opening diameter down to as small as a half of the standard nozzle size. The simulation was applied again to the new inkjet head for optimizing a driving waveform which can eject very small droplets. We predicted, based on the simulation, that the motion of the meniscus in the nozzle oriÞce would enable the discharge of a femtoliter-order droplet. Figure 4 shows a motion of the meniscus in an oriÞce at different times in the Þring cycle. The colors in the Þgure indicate velocity magnitude in the oriÞce during droplet formation. In the Þrst step, the meniscus is greatly pulled into the oriÞce and a concave shape is formed at the center portion of the meniscus converged ink ßow to the center part of the meniscus generates a convex portion where the ßuid velocity is high.In the second step, converged ink ßow to the center part of the meniscus generates a 0.001 0.004 0.065 1.023 1.437 2.806 4.189 8.181