Keon Kuk

A monolithic inkjet print head: DomeJet

This paper describes a thermally driven monolithic inkjet print head, DomeJet, that consists of dome-shaped ink chambers and thin film nozzle guides. Nozzle guides are fabricated by anisotropic etching after PECVD oxide passivation of their side walls. Each of the ink chambers and channels are fabricated by isotropic dry etching process using XeF/sub 2/. Omega or ring type polysilicon heaters are integrated on top of each chamber. Fifty-six nozzles are embedded at two columns on each chip where their pitch is 1/150 inch at each column. A pulse voltage of 2.6 /spl mu/s has been used in initial printing tests, resulting in 36 /spl mu/m diameter dots on a HP transparent film that was placed 1 mm away from the nozzle 18 /spl mu/m wide. The drop volume and velocity are measured to be 4 pl and 12 m/s, respectively. A printed image is successfully obtained using the fabricated head installed to a Samsung inkjet printer.

T-jet: a novel thermal inkjet printhead with monolithically fabricated nozzle plate on SOI wafer

A novel thermal inkjet printhead with monolithically fabricated nickel nozzle plate on SOI wafer has been proposed for the first time. A chamber and a restrictor are implemented on the 40 /spl mu/m thick top-silicon layer, and a nozzle plate covering heater layers are monolithically fabricated on them. Unlike the general back-shooters, the inkjet printhead reported here is a kind of back-shooter, which has a chamber and a restrictor with arbitrary shape by utilizing the silicon dioxide etch-stop layers in the bottom and sidewalls of chamber. Moreover, nozzle plating mold process, followed by placing the chamber underneath the heater layer, is performed on a planar surface, resulting in more uniform and reliable control of nozzle size. The new design was applied for monochrome inkjet printhead, which has 56 nozzles in 2 columns with real 600NPI(nozzle per inch), and showed good performances such as a drop velocity of 12 m/s, a drop volume of 30 pl, and a maximum firing frequency of 12 kHz for single nozzle ejection. From nozzle by nozzle inspection, we observed the uniformity variation of less than 4% in drop speed as well as drop volume. The monolithic fabrication process resulted in a good uniformity and is expected to have superior manufacturing yield to the nozzle assembly process.

A Micromachined Monolithic Inkjet Print Head with Dome Shape Chamber

We describe a thermally driven monolithic inkjet print head, DomeJet, that consists of vertical refill channel and dome-shaped ink chambers. The vertical channel can be fabricated by anisotropic etching without photographic process and it has the same diameter with nozzle. Omega or ring type polysilicon heaters are integrated on top of each chamber. We study the performance of nozzles that are differently designed with respect to heater position and width. A 20 µm nozzle with “C type” heater shows stable ejection performance up to 20 kHz with droplet volume of 3.2 pl. Fifty-six nozzles are embedded at two columns on each chip and the pitch of nozzle is 1/150 inch. A printed image is successfully obtained using the fabricated head installed to a Samsung inkjet printer.

Failure mechanisms in thermal inkjet printhead analyzed by experiments and numerical simulation

This paper presents a failure analysis result for enhancing the reliability of thermal inkjet printhead. A novel inkjet printhead is fabricated using MEMS process, and we analyze the failure mechanism of inkjet head based on detailed experimental observations and numerical simulations. The failures presented in this work showed three primary factors influencing the failure modes and lifetime of printhead; bubble cavitation damage, thermal fatigue, and electromigration of heater. The design modification of micro heater to avoid an early stage of failure by electromigration yields the reliability enhancement of thermal inkjet printhead.

Firing frequency improvement of back shooting inkjet printhead by thermal management

This paper presents a firing frequency improvement of a backshooter type inkjet printhead, DomeJet, with an optimized thermal design. DomeJet is a thermally driven monolithic inkjet printhead that consists of dome-shaped ink chambers and omega-shaped heaters over the chambers. As the firing frequency increases, the residual heat accumulated in heater layer causes unstable droplet ejection. In a roofshooter architecture printhead, a silicon substrate with a layer of silicon dioxide between the substrate and the heater is an excellent thermal reservoir. Compared to a roofshooter, a backshooter type head has disadvantage in heat release since the heater is confined to a thin film which has low thermal conductivity. In this study, two models are proposed. Model I has an aluminum metal layer over the heater that forms heat passage to the substrate, and Model II has thick nickel nozzle plate in contact with the metal layer to enhance heat dissipation capability. The result gives a head being operated at a frequency of 40 kHz.

Lumped Modeling of Crosstalk Behavior of Thermal Inkjet Print Heads

This paper presents a lumped model to predict crosstalk characteristics of thermally driven inkjet print heads. The model is based on a heat conduction equation, an empirical pressure-temperature equation, and a nonlinear hydraulic flow-pressure equation. It has been simulated through the construction of a Kirchhoffian R-L-C network, and subsequently analyzed using SIMULINK and an electronic circuit simulation tool. Using the lumped R-C model, heating characteristics of the head are predicted to be in agreement with IR temperature measurements. The inter-channel crosstalk is simulated using the lumped R-L network. The values of viscous flow resistance, R and flow inertance, L of the inter-channels are adjusted to accord with the 3-D numerical simulation results of three adjacent jets. The crosstalk behaviors of a back shooter head as well as a top shooter head have been investigated. Predictions of the proposed lumped model of the meniscus oscillations are consistent with numerical simulations. Comparison of the lumped model with experimental results identifies that abnormal two-drop ejection phenomena are related to the increased meniscus oscillations because of the more severe crosstalk effects at higher printing speeds. Our model can be used as a design tool for a better design of thermal inkjet print heads to minimize crosstalk effects.Copyright

Self-standing and shape-memorable UV-curing epoxy polymers for three-dimensional (3D) continuous-filament printing

In the development of three-dimensional printable materials for high-speed and high-resolution printing, UV-curing polymers can guarantee fast and precise printing of high performance load-bearing structures, but the injected drops of the monomers tend to spread over the substrates due to their low viscosity. In this study, we imposed the self-standing and shape-memorable capability of an epoxy acrylate (EA) monomer to ensure continuous filamentary 3D printing while maintaining its low viscosity nature. Using octadecanamide (ODA) with EA, strong hydrogen-bond networks (−N−H⋯OC−, −N−CO⋯H–O–, –N–H⋯N–) were additionally achieved in the material system and the developed material distinctively exhibited rheological duality at different processing stages: a low-viscosity liquid-like behavior (viscosity of ∼50 Pa) while passing through the nozzle and a self-standing solid-like behavior (static yield stress of ∼364 Pa) right after being printed. This reversible liquid-to-solid transitional capability was quantified by viscoelastic complex moduli provided a dynamic yield stress (τy,G) of 210 Pa corresponding to the upright stacking up to ∼3.2 cm (3 wt% of ODA). The time (ty,G) required for conformational rearrangement was evaluated to be as fast as ∼10−2 s. After UV curing, the 3D printed layers exhibited no air pockets or weld lines at the stacked interfaces, which could guarantee excellent mechanical performance and structural integrity.

Actuating Performance of Non-Passivated Micro Heaters

Effects of thin film layers have been investigated on the actuating performance of micro heaters. Bubble behaviors on micro heaters were observed experimentally. Nine (9) kinds of tantalum nitride (TaN) micro heaters were prepared to have different thin film layers with same planar size of 22 μm by 22 μm. Step-stress test (SST) showed that maximum endurable voltage levels in the non-passivated heaters are less than 50% of those in the passivated heaters. Bubble test was carried out using de-ionized (DI) water as working fluid in an open pool. The results showed that the non-passivated heaters can produce comparable bubbles with only 20 to 50% of the input energy required for the passivated heaters. However, the non-passivated heaters could be operated only in a narrow range of driving voltage. Applicability of non-passivated heaters as promising micro actuators needs further investigation from the viewpoints of robustness and reliability.Copyright

Improvement of Firing Frequency Limits by Investigation of Ejection Failure Modes in Thermal Inkjet Print Heads

Ejection failure modes determining the firing frequency limit in thermal inkjet heads were investigated from performance experiments and numerical simulations. Dominant failure modes were affected by the flow resistance ratio in the ink flow passages. Optimal flow resistance ratios were obtained experimentally to provide maximum frequency limits for both mono and color inkjet heads. Numerical simulations were performed on the meniscus oscillation, the ejection behavior in consecutive firing, and the cross-talk induced flow. Numerical results supported the importance of flow resistance ratio in maximizing firing frequency limit in both unit nozzle and multi-nozzle firing. Our investigation will help to develop the inkjet print heads of more reliable high-speed performance.Copyright

Visualization of bubbles generated by microheaters with various current density distributions

This work presents an experimental study to analyze the performance of thermal actuators to generate bubbles. The heaters are designed to have different current density distributions based on numerical analysis. The bubbles generated by heaters are compared to each other by visualization techniques. It is shown that the geometries and driving conditions of the heater strongly affect the bubble behaviors. The geometries like arc or trapezoid-shape induce nonuniform current densities and therefore change the driving conditions for generating bubbles. The experimental result reveals that the proper considerations on heater geometry are required to use the heater as a thermal actuator.