Electrohydrodynamic Inkjet Printing Research Articles

3D printing methods for micro- and nanostructures

The physical and physicochemical fundamentals of three-dimensional (3D) micro- and nanoprinting are presented. 3D printing (or additive manufacturing technology) is a process which fabricates structures and devices by depositing material (usually layer by layer) according to a 3D digital model. The methods and results reviewed here are limited to those from micro- and nanoscale fields, which are in demand in the fields of electronics, photonics, and bionics. Special attention is given to methods for fabricating sub-100-nm structures, including single- and two-photon polymerization stereolithography, electrohydrodynamic inkjet printing, and laser-induced forward transfer. The advantages and disadvantages of 3D printing methods are discussed, together with prospects for their development and application.

Mechanisms and modeling of electrohydrodynamic phenomena.

The purpose of this paper is to review the mechanisms of electrohydrodynamic (EHD) phenomenon. From this review, researchers and students can learn principles and development history of EHD. Significant progress has been identified in research and development of EHD high-resolution deposition as a direct additive manufacturing method, and more effort will be driven to this direction soon. An introduction is given about current trend of additive manufacturing and advantages of EHD inkjet printing. Both theoretical models and experiment approaches about the formation of cone, development of cone-jet transition and stability of jet are presented. The formation of a stable cone-jet is the key factor for precision EHD printing which will be discussed. Different scaling laws can be used to predict the diameter of jet and emitted current in different parametrical ranges. The information available in this review builds a bridge between EHD phenomenon and three-dimensional high-resolution inkjet printing.

Designs and applications of electrohydrodynamic 3D printing.

This paper mainly reviews the designs of electrohydrodynamic (EHD) inkjet printing machine and related applications. The review introduces the features of EHD printing and its possible research directions. Significant progress has been identified in research and development of EHD high-resolution printing as a direct additive manufacturing method, and more effort will be driven to this direction soon. An introduction is given about current trend of additive manufacturing and advantages of EHD inkjet printing. Designs of EHD printing platform and applications of different technologies are discussed. Currently, EHD jet printing is in its infancy stage with several inherent problems to be overcome, such as low yielding rate and limitation of stand-off height. Some potential modifications are proposed to improve printing performance. EHD high-resolution printing has already been applied to precision components for electronics and biotechnology applications. This paper gives a review about the latest research regarding EHD used for high-resolution inkjet printing. A starting base is given to help researchers and students to get a quick overview on the recent development of EHD printing technology.

Breakup Length of Electrified Liquid Jets: Scaling Laws and Applications

The instability mechanism of an electrified liquid jet shows different behavior in the viscid (having positive viscosity) and inviscid (showing no resistance to shear stress) regimes, which affects the length of the jet before it breaks up into droplets. Even after much research on electrified jets, a quantitative estimate for the breakup length is still lacking. The authors present a physical model, verified by systematic experiments, that predicts the breakup length in both the viscid and inviscid regimes. This model shows how to attain enhanced quality and resolution in electrohydrodynamic inkjet printing, for example.

Application of PCA-based hybrid methodologies for parameter optimization of E-jet based micro-fabrication process: a comparative study

E-jet or electrohydrodynamic inkjet printing is a direct write, high-resolution, free-form micro- and nano-fabrication process to deposit a wide variety of functional materials on a substrate. The present study tries to tune the process operation control parameters to achieve better functioning of the fabrication process through the optimization route. The average droplet frequency (printing frequency) and average droplet diameter are taken as the major performance characteristics to evaluate the printing process whereas pressure, nozzle substrate gap (stand-off height) and applied voltage are selected as the process control parameters which are to be tuned in the investigated parametric space. Gray relation grade analysis (GRA) and desirability function analysis (DFA) are employed individually to evaluate favorable process environment of E-jet which could satisfy the multiple responses simultaneously with a reasonable trade-off. Principal component analysis (PCA) is utilized to determine the relative importance of the performance characteristics to make the optimization methodology more robust, thereby reducing the dependence on the practitioner’s judgment in the decision-making process. Moreover, a comparative study is also carried out to assess the relative performance of the adopted two hybrid methodologies. It reduces the selection difficulty a decision maker may face while choosing the most appropriate methodology for the E-jet optimization scenario. Signal to noise (S/N) ratio is used as a yardstick for comparison. The result showed that the PCA–GRA has better S/N ratio than PCA–DFA method. It can be concluded that PCA-integrated GRA performs better than the PCA-coupled DFA for E-jet optimization. The identified optimal level combination of process parameters is pressure 1.72 kPa, gap 30 µm and a high level of applied voltage for maximum printing frequency and minimum droplet diameter. Main effect study is used to identify the most influential control parameter in the investigated region. The study revealed that applied voltage is the most significant control parameter for E-jet operation.

High-Resolution E-Jet Enhanced MEMS Packaging

High-resolution electrohydrodynamic inkjet printing has potential to simplify the existing MEMS package fabrication through its additive and digital properties without sacrifices in the I/O density of the package. Initially, the replacement of certain lithographic process steps would lead to decreased material consumption and increased cost-effectiveness of the package fabrication; in long term, the large-scale adoption of additive process steps would enable increasing device customizability and lead to cost-effective fabrication of small batches and even fully tailored device specific MEMS packages. In our work, we have demonstrated the feasibility of this approach by replacing selected lithographic process steps by using high-resolution inkjet technology (Super Inkjet, SIJ Tech.) for metallization of high-density redistribution layers (RDL), metallization of through-silicon-vias (TSV) and fabrication of under bump metallization (UBM). Fully printed high-density redistribution layer (RDL) A fully printed high density RDL in two metallization layers was demonstrated with a combination of Super Inkjet (SIJ) and conventional inkjet printing (DMP-2831). The SIJ was used for the metallization of 5/5 µm W/S high-density circuitry in MET1 and MET2 and DMP for the dielectric layer separating the metallization layers. In order to achieve low resistance conductors with minimal footprint, multilayer printing was employed resulting in a high aspect ratio of over one. The MET1 to MET2 microvia was also additively fabricated by first printing a narrow ~10µm pillar in MET1 with SIJ, then surrounding the pillar with DMP printed dielectric layer and finally connecting the top of the pillar with SIJ printed MET2 conductors. [1] A fully printed RDL increases the customizability of the package fabrication process to a level where each individual interposer on wafer could be fitted with customized RDL to accommodate a specific MEMS device. In addition, the material consumption is reduced significantly since the fully printed RDL does away with global deposition of metal, masking and etching required in standard lithography based metallization. It is also important to notice that these advantages are achieved without sacrificing the density the RDL: the 5/5µm width and spacing for conductors and 10µm microvia diameter are similar to what is achieved with current lithographic processes. Additive metallization of high-density through-silicon-vias (TSV) The sub-femtoliter droplet size of the SIJ and concomitant fast evaporation rate was utilized in rapid metallization of high-density TSVs with top diameter of 23µm and spacing of 10µm. In standard practice CVD, PVD, electroplating or electroless deposition are utilized for metallization of high density TSVs leading to long process time and low level of customizability. The conventional inkjet printing could be used for over 50µm TSV diameters, but it is difficult to achieve stable operation with a 1pl cartridge which could produce small diameter droplets for filling the high density TSVs of 20µm diameter; also the low evaporation rate of the relatively large droplets would prolong the process time since evaporation step would need to be included in the process; in addition, the positioning accuracy of the conventional inkjet printer (e.g. DMP) might not be sufficient for this application. In this regard it was noticed that the SIJ printing has many advantages: first, the sub-femtoliter droplet size leads to fast evaporation rate enabling the metallization of the vias at room temperature without substrate heating or evaporation steps; second, the positioning accuracy of the SIJ stage is under 1µm enabling accurate deposition of the ink in to the TSV; third, since most of the solvent in the droplets is already evaporated during the flight, the chances for creating gas pockets in the filling are reduced. [2] High-resolution inkjet printing for creating smooth UBM The previously discussed approaches for utilizing the high-resolution Super Inkjet printing were restricted to features inside the interposer. However, this fabrication technology also finds utility on the outside of the package, which does not necessarily need such high-density features, but where low surface roughness is important. Highlighting this advantage, a smooth under bump metallization has been fabricated with SIJ on top of a large diameter partially metallized TSV; the TSV has been filled with SIJ printed polymer dielectric. [3] References Laurila et al, “Combination of E-jet and inkjet printing in additive fabrication of multilayer high-density RDL of silicon interposer”, IEEE Trans. Electr. Dev., vol. 64, no. 3, pp. 1217-1224, March 2017 Khorramdel et al, “Metallization of high-density TSVs using Super Inkjet technology”, 2015 IEEE 65th ECTC conf., San Diego, 2015, pp. 41-45 Khorramdel et al, “Inkjet printing technology for increasing the I/O density of 3D TSV interposers”, Microsystems & Nanoengineering, vol. 3, 17002, Apr. 2017

Electrohydrodynamic inkjet printing of Pd loaded SnO2 nanofibers on a CMOS micro hotplate for low power H2 detection

A high-performance low-power micro hotplate (MHP) hydrogen sensor was fabricated through electrohydrodynamic (EHD) inkjet printing technique. Electrospun Pd loaded SnO2 nanofibers with lengths of 250-850 nm were precisely printed on the suspended central part of an MHP with an area of 100 um × 100 um. The printhead in the printing system was a low-cost metallic needle with an inner diameter of 110 um, which was large enough to prevent clogging by the nanofibers. The printing process was observed by a high-speed camera. Small droplets with diameters of 50-80 um were produced at each ejection by providing a high voltage to the metallic needle. It was found that the bridge-type MHPs used in our experiment can promote the positioning precision due to its bound effect to the droplet. In the gas sensing measurement, the Pd loaded SnO2 MHP gas sensor showed a remarkable response to H2 with a low power of only 9.1 mW. The experiment results demonstrate the excellent adequacy of EHD inkjet printing technique to realize effective mass fabrication of MHP gas sensors or sensor arrays.

Machine vision assisted micro-filament detection for real-time monitoring of electrohydrodynamic inkjet printing

Electrohydrodynamic inkjet printing (e-jet printing) is one type of micro/nano scale 3D printing technique that automatically deposits functional materials to form 3D structures on the substrate. Unlike traditional thermal or acoustic inkjet printing, e-jet printing utilizes high electrical forces that enable the ink to overcome surface tension at the tip of micro nozzles. The droplets and filaments coming out from the nozzle have dimensions much smaller than the dimensions of the nozzle, thus printing geometries in micro and nano scale. E-jet printing process parameters could affect the final quality attributes of fabricated constructs. Currently, assessment of these critical geometries and attributes must be performed offline using optical microscopy or scanning electron microscopy. This drawback affected the efficiency of micro/nano printing from translation into industrial practice. The research in this paper focused on fundamental research to enable in-situ monitoring of e-jet printing using a real-time images characterization technique. In conclusion, the study in this paper investigated using machine vision for real-time monitoring of micro/nano scale 3D printing. The method worked well for micro-filament detection in e-jet printing, and may be further implemented into feedback control system of complicated e-jet printing. However, the optical machine vision was limited to micro scale detection. One of the future research topic is to develop nano scale in-situ detection mechanism for e-jet printing.

In-situ real-time characterization of micro-filaments for electrohydrodynamic ink-jet printing using machine vision

Electrohydrodynamic ink-jet printing (e-jet printing) is one of micro/nano scale 3D printing techniques that automatically deposit functional materials to form 3D structures on the substrate. Unlike traditional thermal or acoustic inkjet printing, e-jet printing utilizes high electrical forces that enable the ink to overcome surface tension at the tip of micro needles. The filaments/droplets coming out from the needle have dimensions much smaller than the dimensions of the nozzle, thus printing geometries in micro and nano scale. Process parameters in e-jet printing could affect the final quality attributes of fabricated constructs. Currently, assessment of these critical geometries and attributes must be performed offline using optical microscopy or scanning electron microscopy. This drawback affected the efficiency of micro/nano printing from translation into industrial practice. The research in this paper focused on fundamental research to enable in-situ monitoring of e-jet printing using a real-time image characterization technique. In conclusion, the study in this paper investigated using machine vision for real-time monitoring of micro scale 3D printing. The method worked well for characterization of micro-filaments, and may be further implemented into feedback control system of complicated e-jet printing. However, the optical machine vision was limited to micro scale detection. One of the future research topic is to develop nano scale in-situ characterization mechanism for e-jet printing.

Design, Development and Experimental Investigation of E-jet Based Additive Manufacturing Process

Electrohydrodynamic ink jet (E-Jet) printing is a non-contact mask less micro fabrication technique, where the flow of functional material in the form of ink is induced by electric field, rather than acoustic or thermal form of energy. This approach can be extended for high-resolution printing applications. In this paper an E-jet based additive manufacturing fabrication approach is demonstrated where user can print various functional materials with different ink substrate combinations. Limitations of the conventional ink jet technology, applications of this new emerging technology along with its potential market have also been discussed. A detailed description of the E-Jet printing technology is demonstrated. The effects of different operating parameters on process output variables have also been investigated in this study. Applied potential difference, stand-off height between the nozzle and the substrate and ink flow rate have been taken as operating parameters and printed droplet diameter and ejection frequency defines the printing performance characteristics. Experimentation results validate the potential of the aforementioned technology i.e. high resolution patterning of the functional material with micron range feature size.

Parametric Optimization of E-Jet Based Micro Manufacturing System through Hybrid Taguchi Methodology

Electrohydrodynamic inkjet (E-Jet) printing is a non-contact mask less additive fabrication technology to print wide variety of functional materials in micro/nano scale. The material wastage in this manufacturing technology is almost avoided, which is the upmost importance while patterning highly expensive material. The printing performance depends on several process operational variables like applied voltage, stand-off height, nozzle diameter, applied pressure etc. By tuning these process variables through optimization route improved printing performance can be achieved. In this present study, an attempt has been made to optimize the printing performance characteristics namely, printed droplet diameter and ejection frequency with the help of Weighted Principle Component Analysis (WPCA) coupled with Taguchi methodology. Applied voltage between the nozzle and the substrate, applied pressure to the ink chamber, and the nozzle stand-off height taken as process parameters which are to be tuned. WPCA convert the multi-response problem into single objective problem, thus enabling the employability of Taguchi’s robust design approach and also the correlation among the responses if exists well taken care of. The study proposes favorable operational condition of the process and also highlighted the features of the proposed synergistic combination of WPCA with Taguchi method for predicting the optimal setting of process parameters.

Fabrication and electrical characterization of multi-layer capacitive touch sensors on flexible substrates by additive e-jet printing

Current consumer electronics, in particular touch displays and flexible electronics, were limited by the properties of existing commercial transparent conductor materials used as electrodes both in flat panel display and capacitive touch sensors. In this paper, an alternative fabrication technique using silver nanoink that can be used for rapid prototyping of high-resolution electrode arrays to replace indium tin oxide (ITO) for flexible electronics was presented. By direct printing silver nanoparticles on flexible substrates, capacitive touch sensors were fabricated onto polyethylene terephthalate (PET) film. Experiments were conducted to study the feasibility of electrohydrodynamic inkjet printing (e-jet printing) of high-resolution electrodes for touch sensors. Sensitivity of sub-20μm capacitance sensor array was investigated in the study for droplet and humidity detection applications. The rapid prototyping method makes a significant impact in enabling simultaneously (1) customized and flexible touch sensors, (2) cost-effective manufacturing, and (3) high resolution and good sensitivity. The presented techniques can be used for the on-demand fabrication of customized conductive patterns for flexible and wearable electronics.

Combination of E-Jet and Inkjet Printing for Additive Fabrication of Multilayer High-Density RDL of Silicon Interposer

The additive nature and high resolution of electrohydrodynamic inkjet (E-jet) printing can be utilized for manufacturing micrometer scale conductive tracks such as those required in the high-density redistribution layers (RDLs) of silicon interposers used in electronics packaging for 3-D integration. Compared to the current lithographic fabrication method, this approach promises to increase the customizability of the process and reduce the amount of waste materials, thereby lowering the costs and the environmental impact of the manufacturing process. In this paper, multilayer interdigitated capacitor and meander resistor structures with 5/5 $\mu \text{m}$ conductor width/spacing are used to demonstrate the feasibility of E-jet printing of high-density multilayer RDLs. A sheet resistance of 28.5 $\text{m}\mathrm {\Omega }$ /square was achieved for the first metallization layer (MET1) conductors and 313.2 $\text{m}\mathrm {\Omega }$ /square for the MET2 conductors. The thickness of the conductors was 6.9 $ {\mu }\text{m}$ for MET1 and 5.4 ${\mu }\text{m}$ for MET2.

High-resolution electrohydrodynamic inkjet printing of stretchable metal oxide semiconductor transistors with high performance.

As demands for high pixel densities and wearable forms of displays increase, high-resolution printing technologies to achieve high performance transistors beyond current amorphous silicon levels and to allow low-temperature solution processability for plastic substrates have been explored as key processes in emerging flexible electronics. This study describes electrohydrodynamic inkjet (e-jet) technology for direct printing of oxide semiconductor thin film transistors (TFTs) with high resolution (minimum line width: 2 μm) and superb performance, including high mobility (∼230 cm2 V-1 s-1). Logic operations of the amplifier circuits composed of these e-jet-printed metal oxide semiconductor (MOS) TFTs demonstrate their high performance. Printed In2O TFTs with e-jet printing-assisted high-resolution S/D electrodes were prepared, and the direct printing of passivation layers on these channels enhanced their gate-bias stabilities significantly. Moreover, low process temperatures (<250 °C) enable the use of thin plastic substrates; highly flexible and stretchable TFT arrays have been demonstrated, suggesting promise for next-generation printed electronics.

The Effect of External Voltage on Pulsed Electrohydrodynamic Printing

Picoliter or femtoliter droplets generated with high frequency is very useful for many applications in different valuable fields, such as microarray generation, drug delivery or biochips. Pulsed electrohydrodynamic inkjet printing (Pulsed EHDP) is an advanced technology which is used for generating fine droplets in recent years. Apparently, the deposition frequency of droplets is a key parameter, and it is influenced by various factors. In this paper, the effect of duty cycle and voltage frequency on deposition frequency is investigated in the pulsation mode. A simple scaling law is proposed to demonstrate relationships between these two parameters and deposition frequency. Results show that volume of the liquid accumulated at the apex of the nozzle changes during the non-electric field period, which affects taylor cone deformation time and deposition frequency. High deposition frequency can be achieved by increasing the duty cycle up to 0.7, and choosing voltage frequency between 10Hz to 40Hz. These analysis is valuable for understanding Pulsed EHDP deeply and fabricating the micro/nanoapplications.

Direct printing of reduced graphene oxide on planar or highly curved surfaces with high resolutions using electrohydrodynamics.

Electrohydrodynamic inkjet printing of reduced graphene oxide (RGO) is de-monstrated to form complex geometric devices with high resolution (line width ≈ 5 mm). Both planar and highly curved surfaces (radius of curvature ≈ 60 mm) can be used as substrates. Demonstrations of counterfeit coin recognition using RGO patterns and all-printed RGO transistors suggest substantial promise for applications in security and electronics.

Dielectric Characterization of Pigment Inks for Electrohydrodynamic Jet Printing

This article presents a characterization method that could be useful for evaluating the performance of ink in electrohydrodynamic jet printing. The ink contains pigment particles and polymer (stabilizer for pigments) and has a low viscosity and a low elasticity. Depending on the medium (hydrocarbon for oil-based inks and glycol ether for polar solvent-based inks) and the type of pigment (exhibiting different colors), vast differences were found in the dielectric properties even when the viscosities of the inks were almost identical. These differences between the two series of inks were related to differences in the ion concentration n and ion mobility μ estimated from an analysis of the complex dielectric constant e* (= e′ – ie″) on the basis of Macdonald theory. These differences between the polar solvent- and oil-based inks are discussed in relation to the performances of the inks in electrohydrodynamic inkjet printing.

Optimization of Experimental Parameters To Determine the Jetting Regimes in Electrohydrodynamic Printing

The harmony of ink and printing method is of importance in producing on-demand droplets and jets of ink. Many factors including the material properties, the processing conditions, and the nozzle geometry affect the printing quality. In electrohydrodynamic (EHD) printing where droplets or jets are generated by the electrostatic force, the physical as well as the electrical properties of the fluid should be taken into account to achieve the desired performance. In this study, a systematic approach was suggested for finding the processing windows of the EHD printing. Six dimensionless parameters were organized and applied to the printing system of ethanol/terpineol mixtures. On the basis of the correlation of the dimensionless voltage and the charge relaxation length, the jet diameter of cone-jet mode was characterized, and the semicone angle was compared with the theoretical Taylor angle. In addition, the ratio of electric normal force and electric tangential force on the charged surface of the Taylor cone was recommended as a parameter that determines the degree of cone-jet stability. The cone-jet became more stable as this ratio got smaller. This approach was a systematic and effective way of obtaining the Taylor cone of the cone-jet mode and evaluating the jetting stability. The control of the inks with optimized experimental parameters by this method will improve the jetting performance in EHD inkjet printing.

The Effect of Substrate on Continuous Electrohydrodynamic Printing

In this paper, the behavior of ejected jet is studied at three different substrates (conductive, semiconductor and dielectric) in continuous electrohydrodynamic inkjet printing mode. Because the polarization charges will accumulate at the surface of the substrate in a short nozzle-to-collector distance, one can observe that the different flight behavior in the space. Results show that the substrate has little effect on the threshold voltage and the relaxation time of the substrate can be used to indicate the behavior of the jet. When the lifetime of the jet is larger than the relaxation time of the substrate, the jet can be deposited on the substrate regularly. Based on this guideline, a designed parallel lines and grid pattern are fabricated with several micrometers on a silicon substrate. These show that the designed pattern could be fabricated in a controllable jet. In the future, this method is able to be utilized to fabricate the mirco/nano-devices.

Investigation on switching behavior of ZrO2 thin film for memory device applications

In this paper, we have investigated the resistive switching behavior of nanostructured zirconium oxide (ZrO2) thin film deposition by spin coating. The metal (silver) electrodes were patterned by electrohydrodynamic inkjet printing technique. The X-ray diffraction and Fourier transform infra-red spectra confirmed that the presence of monoclinic phase in the as deposited ZrO2 thin film. The field emission scanning electron microscopic image revealed the uniform deposition of ZrO2 thin film with spherical morphology. The as-fabricated Ag/ZrO2/Ag memory device exhibited the characteristic bipolar resistive switching behavior under consecutive dc sweep. The possible mechanism of the bipolar resistive switching has been discussed in detail. The endurance and retention analysis of the fabricated device revealed the stability of the device. Our results ensure the promising applications of ZrO2 thin film in the memory device applications.