Ink-Jet Metalization Dalla Costa Giovanni, Honkala Salomon, Kalliala Olli, Multaharju Miikka

Contents Ink jetting Nanoparticle properties Sintering Process Electrical analysis

Jetting the ink Generally used print heads: Thermal bubble ink-jet Piezoelectric ink-jet Continuous stream ink-jet Lin,L. Bai,X.

Generally used print heads: Thermal bubble ink-jet Piezoelectric ink-jet High nozzle density→ compact devices, low printhead costs Ink fluid limitations→ vaporizable ink Coating formation on the heater→ degrades efficiency, reduces life time Printheads expensive Wide range of ink fluids Controllable jetting mechanism High reliability Long life time Professional and industrial applications Ezzeldin. van den Bosch. Weiland.

Piezoelectric DOD print heads Bend-mode Squeeze-mode Push-mode Shear-mode Cummins. Desmulliez.

Ink jetting mechanism with piezoelectric actuator An electric voltage is applied to the actuator The channel enlarges and a negative pressure is created Negative pressure waves advance towards the reservoir and the nozzle Pressure wave at the reservoir reflects as a positive wave (open end) Pressure wave at the nozzle reflects and causes the meniscus to retract (closed end) Reflected wave from the reservoir reaches to the middle just when the actuator decreases the volume of the channel thus creating an amplified wavefront Amplified wave results the drop injection Ezzeldin. van den Bosch. Weiland.

Actuation pulse control To avoid crosstalk and residual vibrations that causes drop velocity and volume variation! Crosstalk influences neighboring channels changing their pressure and volume. Residual vibrations of individual channel affect in sequential ejections. Model-based (differential wave equations) feedforward control Model-free (optimized velocity) feedforward control Ezzeldin. van den Bosch. Weiland.

Actuation pulse control Normal actuation pulse leads to variation in drop volume and its velocity. Which consequently affects accuracy and printing quality. Using feedforward control reduces such variations. Ezzeldin. van den Bosch. Weiland.

One nozzle jetting test Experiment shows a single nozzle ejecting 16 drops with 48 kHz. Optimized pulse results in congruent lines excluding the first and the satellite. Standard pulse produces drops traveling with different speeds and many of them are merged and misplaced. Multichannel control with asynchronous actuation needed to minimize crosstalk. Ezzeldin. van den Bosch. Weiland.

Minimum line width Some variables has to be set beforehand: Cummins. Desmulliez. Some variables has to be set beforehand: 1 < 1/Oh < 10 η ≈ 7…20*10-3 Pas σ ≈ 25…35*10-3 N/m v ≈ 4…8 m/s ρ ≈ 1300 kg/m3 Pixel/D ≈ 1,5 We want D to be as small as possible.

Nanoparticle properties The size and crystallographic structure of the nanoparticle has an essential role in determining the catalytic properties Different crystal structures (and faces) such as nanoplates, nanocubic and near-spherical has different kind of properties For silver example it has shown that the highest rate of reaction is achieved with nanocubic structure Development towards increase of the more-reactive crystal planes and a decrease in less-reactive planes. Other important properties than crystal structure are the ratio of surface area high vs (improves sintering in low temperatures), chemical stability, low chemical reactivity and high conductivity.

Nanoparticle properties: Shape factor example (silver) The catalytic activity of the nanoparticles greatly depends on the crystal planes that the nanoparticles expose. For example in silver nanocubes shows higher activity than near-spherical or nanoplate particles because of their more reactive [100] planes. On the other hand truncated triangle form nanoplate (A) has the most uniform structure which decreases the amount of grain boundaries  increase to conductivity Fig.1. TEM images of A)truncated triangular nanoplates, B)near-spherical nanoparticles, C)nanocube [R. Xu, D.S. Wang, J.T. Zhang, Y.D. Li, Shape-dependent catalytic activity of silver nanoparticles for the oxidation of styrene, Chem. Asian J. 1 (2006) 888–893.]

Nanoparticle properties: Metals for nanoparticles Ag and Au are widely used due to their high chemical stability, low chemical reactivity and high conductivity. Copper and nickel inks have also been produced but their tendency to oxidise can affect the lifetime of the ink or require the use of additional specialized coatings in printing in inert atmospheres Ag nanoparticles can be sintered with a relatively low temperature( 150-200˚C) when printed on substrates. High surface area vs. volume is needed. For example Au nanoparticles with diameters less than 5nm are predicted to melt at 300-500˚C

Challenges in nanoparticle properties Low sintering temperatures needed due to use of flexible substrates Metals have the tendency for agglomeration which further leads to increase in viscosity. Clogging can be prevented by using specialised polymer coatings on the nanoparticles. However, removing of the surface needs high sintering temperatures which makes it unsuitable for flexible substrates. Organic coating can be removed by photonic or microwave flash sintering at low temp. As the nanoparticle size decreases so does the conductance as well. This means that the conductivity of sintered nanoparticle rarely matches to bulk form. Because of the flexible electronic device polymer substrate, the sintering temperature must be lowered to 100˚C. In this temperature it is not clear yet how low resistance can be obtained in the printed film.

Sintering Consolidate the metallic particles in order to create a continuous conductive path Removing of organic solvent or binder Diffusion process Parameters Particle size and shape Process environment Time and Temperature

Mechanism

Thermal Tm Nanoparticles < Tm Bulk T needed << Tm up to 500˚C less T needed << Tm 100-300˚C Increase of sintering time is useless Long time is needed

Q. Huang et al. / Applied Surface Science 258 (2012) 7384– 7388

Light curing Use of a light source: IR, Camera-flash, Laser Laser is a slow process Difference of light absorption is important 250 times difference in temperature increase Substrate interaction with light is important

D. Tobjörk et al. / Thin Solid Films 520 (2012) 2949–2955

Q. Huang et al. / Applied Surface Science 258 (2012) 7384– 7388

K.C. Yung et al. / Journal of Materials Processing Technology 210 (2010) 2268–2272

Plasma Need of vacuum chamber Not for thick layer Long time process J. Mater. Chem., 2009, 19, 3384–3388

Sheet resistance Resistance in 3D conductor: 𝑅=𝜌 𝐿 𝐴 =𝜌 𝐿 𝑊𝑡 Combining resistivity with thickness: 𝑅= 𝜌 𝑡 𝐿 𝐴 = 𝑅 𝑠 𝐿 𝑊 [ohms/square; Ω/□]

Greek-cross test structure 𝑅 (0°) = 𝑉 𝐷𝐶 − 𝑉 𝐶𝐷 𝐼 𝐴𝐵 − 𝐼 𝐵𝐴 𝑅 (90°) = 𝑉 𝐴𝐷 − 𝑉 𝐷𝐴 𝐼 𝐵𝐶 − 𝐼 𝐶𝐵 𝑅 𝑠 = 𝜋 ln⁡(2) 𝑅 (avg) (Van Der Pauw, 1958) Enderling et al. (2006), “Sheet resistance measurement of non-standard cleanroom materials using suspended Greek cross test structures”, IEEE Transactions on Semiconductor Manufacturing, Vol. 19 No. 1, pp. 2-9.

High-frequency characterization Many HF-applications (RFID, antennas…) Losses due to microstructure at high frequencies , depending on printing parameters, inks and substrates Transmission-line test structures Use vector network analyzer to determine scattering parameters Pynttäri et al. (2010), ”Application of Wide-Band Material Characterization Methods to Printable Electronics”, IEEE Transactions on Electronics Packaging Manufacturing, Vol. 33 No. 3, pp. 221-7.

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