DESIGN & FABRICATION OF ELECTROSTATIC INKJET HEAD USING SILICON

MICROMACHINING TECHNOLOGY

Contents:

• Introduction• Scope • Objective• Design & Simulation• Fabrication• Experimental Set up• Conclusion• References

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INTRODUCTION

• Ink-jet printing technique is very attractive for forming micro-size patterns for flat panel displays (FPDS),• Printing circuit board (PCB), • Semiconductor, • Biological, • Optical, • Sensor devices

• due to its low temperature process, direct writing, and rapid photolithography process

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• The inkjet printing mechanisms has been used are

• Thermal method

• Piezoelectric method

• Electrostatic method

INTRODUCTION Cont..

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• Thermal and piezoelectric inkjet printing are based on pushing out the liquid in a chamber through a nozzle by actuators, such as thermal bubble and piezoelectric actuators.

• Thermal bubble actuator has the heat problem when the array of nozzle make in a large area

• Piezoelectric actuator is difficult to make droplet smaller than nozzle size . The head consisting of piezoelectric or a heater element has the disadvantage of the complex structure

INTRODUCTION Cont..

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• Electrostatic inkjet are based on protruding through an orifice induced by an electric field forms a meniscus called the Taylor cone.

• Also it can separate from the meniscus tip as fine droplets much smaller than the orifice diameter.

• Electrostatic inkjet has a greater advantage than piezoelectric inkjet and thermal inkjet in the ability to eject fine droplets and in the simplicity of structure

INTRODUCTION Cont..

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SCOPE

This paper discusses about the design and fabrication of optimized geometry structure of Electrostatic Inkjet Head.

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Objective

To design and fabricate inkjet heads to achieve an effective micro dripping ejection, to provide concentrated electric force on nozzle and pole tip.

DESIGN & SIMULATION

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• They designed and fabricated the following electrostatic inkjet heads, • electrostatic inkjet heads, hole type • electrostatic inkjet heads, Pole type

Hole type Inkjet Head Pole type inkjet head

DESIGN & SIMULATION

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• This structure consists of

1. Reservoir,

2. Nozzle

3. Conductive pole to concentrate electric field.

DESIGN & SIMULATION Cont…..

Hole type Inkjet Head

Hole type Inkjet Head

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Schematic of the proposed Pole type inkjet headelectrostatic inkjet head.

1. This structure consists of glass electrode part

2. Nozzle part with a reservoir.

Fig. 3. The structure of the proposed pole type electrostatic inkjethead.

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Δ .Є Δ.Ф = -þ Ē= - ΔФ Where

• Ē -electric field vector

• -Ф- electric potential

• Є- permittivity

• þ - charge density

• Micro droplet is formed from the liquid meniscus due the induced electric field.

• The electric field is obtained by solving the following governing equations. (These equations are solved by FEM method)

DESIGN & SIMULATION Cont…

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• The inkjet head utilizes electrostatic forces that act between electrode and nozzle tip.

• The electrostatic forces are created when a voltage is applied electrode and head bottom.

• The surface of the inkjet head becomes wet with liquid after applying an electric field.

• Then charged liquid is separated from the inkjet head tip as fine droplets.

Mechanism of Spray formation

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• When the force induced by an electric field on the inkjet head tip is stronger than the resultant force of

• surface tension, • ink viscosity and • conductivity,

• The meniscus called Taylor cone is generated and fine droplets is separated from the head by Coulomb force.

Mechanism of Spray formation Cont..

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Need For Simulation

• In order to verify effect of geometry shape, we simulate electric field intensity according to the head structure.

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Electric Field Simulation of a Hole type inkjet head when DI water is used

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Electric Field Strength Vs. Nozzle height of a Pole type inkjet head

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Result of the Electric field simulation of the hole type inkjet head

Electric field simulation of the hole type inkjet head showed the following results

• The electric field strength increases linearly with increasing height of the micro nozzle.

• Also, as the height of the nozzle increases, the electric field along the periphery of the meniscus can be more concentrated.

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• In order to find optimal structure and demonstrate concentration of electrostatic force at the pole edge, pole type inkjet head is simulated using FEMLAB.

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Result of the electric field simulation used the DI water as the liquid solution.

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Result of the Electric field simulation of the hole type inkjet head

• The structure of the nozzle inner diameter = 80 μm• Thickness, 20 μm • Pole diameter are, and 40 μm, • The pole height is 50 μm.

As the result, the concentration of electrostatic force was shown at the conductive pole edge.

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FABRICATION

• Electrostatic inkjet head consist of 2 layers

• Glass top layer• Silicon bottom layer.

• Fabrication is done using

• thermal-oxidation • silicon micromachining technique .

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Fabrication process Cont…

• SiO₂ layer on silicon wafer.

• Oxide(deep Si etch mask)

patterning and deep Si etching.

• Reservoir patterning on the

bottom silicon wafer.

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• Deep Si etching.

• Deep Si etching for pole

formation.

• SiO₂ removal by HF solution.

Fabrication process Cont…

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Fig. 7. Fabrication Process : (a) SiO2 layer on silicon wafer, (b) Oxide(deep Si etch mask) patterning and deep Si etching, (c) reservoir patterning on the bottom silicon wafer, (d) Deep Si etching, (e) Deep Si etching for pole formation, (f) SiO2 removal by the HF solution.

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Photomicrograph of Hole & Pole type nozzle

Hole type Nozzle

Pole type Nozzle

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SEM Images of Nozzle

Hole type Nozzle

Pole type Nozzle

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EXPERIMENTAL SET UP

Fig. 11. The schematic of the experimental system.

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• This system consists of

• The head system,• High speed camera, • Micro syringe pump,• Power,• Computer

EXPERIMENTAL SET UP Cont….

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• To visualize droplet ejection. high speed camera (IDT XS-4) with a micro-zoom lens and a halogen lamp was used

• The high speed camera can image 5000 frames per second at a 512 x 512 resolution with a micro-zoom lens and a LED light source were used.

• .

EXPERIMENTAL SET UP Cont….

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• A high voltage power supply (maximum voltage of 3.0 kV) was used with a relay switch to control electrostatic field.

• The liquid have been supplied to the nozzle with constant velocity by micro syringe pump and the voltage has been provided to the upper electrode

EXPERIMENTAL SET UP Cont….

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• The jetting mode depends on

• the applied voltage,

• the flow rate,

• liquid properties such as

• Electric conductivity, • surface tension, • and viscosity.

EXPERIMENTAL SET UP Cont….

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• To make an experiment on the micro ejection of the electrostatic inkjet head, the conductive liquid of the mixture of D2O, SDS, and micelle-suspended Carbon Nano Tube (5 %wt SWNT) solution is used as ink.

• The outer diameter of the nozzle is 50 μm • The gap between the upper electrode • The nozzle orifice is set about 800 μm. • The constant flow rate by a micro pump is kept at

0.1 μl/min.

EXPERIMENTAL SET UP Cont….

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Results • When flow rate is made constant at 0.1µl/min and 1.7KV is

supplied, a droplet 80µm is obtained.

Fig. 12. Images taken with high-speed camera showing event ofmicro ejection from nozzle of inkjet head.

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• When flow rate is made constant at 0.1µl/min and the supplied voltage is kept at 2.5 kV. The droplet diameter ejected from the nozzle tip is measured about 10 μm.

Results Cont……

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Conclusion

• This paper discussed about the design and fabrication of the following electrostatic inkjet heads.

• electrostatic inkjet heads hole type• electrostatic inkjet pole type

• It was fabricated using thick-thermal oxidation and silicon micromachining technique such as

• the deep reactive ion etching (DRIE) • chemical wet etching process.

• The fabrication process used is very simple and reproducible..

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REFERENCES• Design and Fabrication of Electrostatic Inkjet Head using Silicon Micromachining

Technology Youngmin Kim*, Sanguk Son*, Jaeyong Choi*, Doyoung Byun**, and Sukhan Lee* JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.8, NO.2, JUNE, 2008

• Yuji Ishida, Kazunori Hakiai, Akiyoshi Baba and Tanemasa Asano, “Electrostatic Inkjet Patterning Using Si Needle Prepared by Anodization,” Japanese Journal of Applied Physics, Vol. 44, No. 7B, pp. 5786-5790, July 2005.

• S. Lee, D. Byun, S. J. Han, S. U. Son, Y. J. Kim, H. S. Ko, “Electrostatic Droplet Formation and Ejection of Colloid,” MHS, July 31, 2004.

• S.U.Son, Y.M.Kim, J.Y.Choi, S.H.Lee, H.S.Ko, and D.Y.Byun, “Fabrication of MEMS Inkjet Head for Drop-on-Demand Ejection of Electrostatic Force Method,” Trans. KIEE. Vol.56. August, 2007.

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