Artur Lutfurakhmanov

A Review on Aerosol-Based Direct-Write and Its Applications for Microelectronics

Aerosol-based direct-write refers to the additive process of printing CAD/CAM features from an apparatus which creates a liquid or solid aerosol beam. Direct-write technologies are poised to become useful tools in the microelectronics industry for rapid prototyping of components such as interconnects, sensors, and thin film transistors (TFTs), with new applications for aerosol direct-write being rapidly conceived. This paper aims to review direct-write technologies, with an emphasis on aerosol-based systems. The different currently available state-of-the-art systems such as Aerosol Jet CAB-DW, MCS, and aerodynamic lenses are described. A review and analysis of the physics behind the fluid-particle interactions including Stokes and Saffman force, experimental observations, and how a full understanding of theory and experiments can lead to new technology are presented. Finally, the applications of aerosol direct-write for microelectronics are discussed.

Capillary-based liquid microdroplet deposition

Liquid droplet deposition through a capillary onto a substrate is studied. The application of pressure into the capillary causes a liquid meniscus to form at the outlet. Touching the substrate with the liquid meniscus and subsequent capillary retraction gives liquid deposition on the substrate. Theoretical and experimental studies of the steady liquid bridge structure between the capillary and substrate identified the range of parameters when deposition of small droplets (no blot) can be performed. Experiments revealed that in this range of parameters the size of deposited liquid droplets is less than 10%–15% of inner diameter of the capillary.

Micro Cold Spray Direct Write Process

Gas dynamic cold spray was first discovered in the 1980s and has since been used as a surface coating process for depositing metals, metal-ceramic composites, metal-carbon nanotube composites and other composite materials onto both flexible and rigid substrates. We recently developed a focused cold spray material deposition tool termed Micro Cold Spray (MCS). MCS is a direct-write tool applicable for printed electronics and has been used to print conductive trace patterns as thin as 50 μm wide using copper, aluminum and tin micro powders. Unlike conventional aerosol processing at 10–100 m/s, aerosol particles in the MCS process are accelerated to speeds greater than 500 m/s. In this paper the possibility to accelerate, focus, collimate, and deposit aerosol particles is theoretically explored using a finite difference approximation method to simulate the flow of Helium through a symmetric converging-diverging nozzle of throat diameter 200 μm. A Lagrangian particle tracking algorithm is used to calculate the particle trajectories and corresponding velocities. This paper presents a comparison of the effect of Stoke’s drag force and Saffman’s lift force on the trajectory and velocity of copper particles 3 μm in diameter.Copyright

Aerosol Flow Through a Converging-Diverging Micro-Nozzle

Abstract Cold spray is a material deposition process used to deposit metallic features for use in applications such as corrosion prevention, dimensional correction and repair, and wear resistant coatings. Micro Cold Spray Direct Write (MCS-DW) is a variant of the cold spray process in which metal particles are accelerated and focused to print fine features on flexible and rigid substrates with no postprocessing required. This paper presents results of numerical studies on the flow of 2 µm and 6 µm diameter silver particles through a converging-diverging micro nozzle with helium gas. The flow of helium was simulated by solving Navier Stokes equation using commercial software. The trajectory and velocity of the aerosol particles were determined using a Lagrangian particle tracking algorithm with a combination of Stokes drag force and Saffman lift force acting on the particles. A comparison of the effect of different corrections applied to Stokes and Saffman forces as well as the effect of Magnus force on the trajectory and velocity of aerosol particles is studied. The effect of particle rotation creating Magnus force is found negligibly small compared to Saffman force for particles of 2 µm diameter, however, the effect is found to be significant for particles of 6 µm diameter. A proposed converging-diverging nozzle is shown capable of accelerating silver particles to velocities as high as 600 m/s and enables aerosol beam widths as thin as 50 µm.

High-Speed Aerosol Flow Through Micro-Nozzles for Direct-Write Processes

Aerosol direct-write printing for mesoscale features has been commercially available since around 2002 from Optomec®. We have developed variances to this process first in Collimated Aerosol Beam-Direct Write (CAB-DW) for printing sub-10 μm features and in Micro Cold Spray for printing with solid metallic aerosols. These deposition tools offer extensive uses, but are still limited in certain applications by either line widths or the amount of overspray. Modeling of aerosol flow through micro-nozzles used in these applications yields a greater understanding of the focusing of these aerosol particles, and may provide a vehicle for new nozzle designs which will further enhance these tools. Recent modeling applied both Stokes and Saffman force to the aerosol particles. Under certain conditions particle rotation and Magnus force may also be necessary to accurately predict the aerosol particles. In this paper we will present our recent results of high-speed flow of 1–10 μm diameter aerosol particles through micro-nozzles in which the model includes all three forces (Stokes, Saffman, Magnus) of fluid-particle interaction, and a comparison of these results to experiments.Copyright

Experimental Characterization of Aerosol Flow Through a Micro-Capillary

Aerosol flow through a long and tapered micro-capillary (MC) for direct write (DW) technology is typically used with small particles of sizes ranging from 0.2 μm to 10 μm at velocities up to 100 m/s. Earlier research showed that the particles coming through a long MC experience Saffman force that moves the particles towards the center of the beam other than the geometric convergence (due to Stokes drag); thus creating a collimated aerosol beam. It was also established that the additional Saffman force becomes more effective with certain particle diameters and velocities. Therefore, for experimental validation, it is important to accurately measure the particle size distribution and velocities coming out of the long MC. However, the current sizing methods are incapable of measuring particles less than 5 μm due to optical limitations. The current paper presents results using a micro-shadowgraphy system from LaVision Inc. to characterize the flow field. A modification of the particle-sizing algorithm is proposed to measure particles of sub-micron sizes. The modified algorithm can be used to accurately size particles of 1μm diameter.Copyright

Aerosol Flow Through a Micro-Capillary

A combined theoretical/experimental study of micron size aerosol flows through micro-capillaries of diameter about 100 μm and length about 1 cm is presented. It is shown that under proper conditions at a relatively high velocity of about 100 m/s such an aerosol flow reveals a new manifestation of microfluidics: the Saffman force acting on aerosol particles in gas flowing through a micro-capillary becomes significant thereby causing noticeable migration of particles toward the center line of the capillary. This finding opens up new opportunities for aerosol focusing, which is in stark contrast to the classical aerodynamic focusing methodologies where only particle inertia and the Stokes force of gas-particle interaction are typically used to control particle trajectories. A mathematical model for aerosol flow through a micro-capillary accounting for complicated interactions between particles and carrier gas is presented. This model describes the experimental observables obtained via shadowgraphy for aerosol beams exiting micro-capillaries. It is further shown that it is possible to design a micro-capillary system capable of generating a Collimated Aerosol Beam (CAB) in which aerosol particles stay very close to a capillary center line. The performance of such a CAB system for direct-write fabrication on a substrate is demonstrated. The lines deposited by CAB for direct-write fabrication are shown to exhibit widths of less than 5 μm — superior to ink-jet. Materials deposition based upon directed aerosol flow has the potential of finding application in the fields of flexible electronics, sensors, and solar cells. In this paper, the genesis of a new materials deposition method termed Collimated Aerosol Beam Direct-Write (CAB-DW) is discussed.Copyright

Advances in Aerosol Direct-Write Technology for Fine Line Applications

Aerosol deposition tools including Aerosol Jet™ and Collimated Aerosol Beam Direct-Write (CAB-DW™) have been used to create fine metallic traces with line widths as thin as 5 μm by focusing and accelerating aerosol particles through a nozzle and onto a substrate. These tools excel at applications requiring a non-contact method of rapid prototyping. We developed the CAB-DW system after an understanding of the fluid-particle interactions inside the Aerosol Jet system was realized. CAB-DW offers both narrower line widths and reduced overspray due to its 3-component converging-diverging-converging nozzle system. The details behind how this system was designed, subsequent progress in aerosol focusing designs, and applications of aerosol deposition systems will be discussed. In addition, a comparison of these aerosol direct-write systems to other direct-write systems and the future prospects of these tools will be presented.Copyright

Bridge Dynamics for Capillary-Based Liquid Deposition

A new capillary-based lithography technique of liquid droplet deposition is further developed. Main advantage of this method in comparison with others techniques is that it is non-invasive both to the substrate and to the writing tip. The method is studied both theoretically and experimentally. To adequately describe bridge dynamics between the capillary and the substrate, proper boundary conditions must be set in the model for the liquid-surface interface. Based on literature review, two laws of contact line motion are identified: Tanner’s law and Blake’s equation. These two approaches are tested in multiple experiments with different retraction speeds from 3 microns/s to 300 microns/s. Analysis of the experimental data show that both Tanner’s and Blake’s equation can describe the correlation between the contact line velocity and the dynamic contact angle. In addition, both laws are employed in the direct numerical simulation of the bridge dynamics using 3D spectral boundary element method. Modeling results are compared with experimental data and show good agreement.Copyright

Spectral Boundary Element Method for Liquid Bridge Dynamics Between Capillary and Substrate

Micro/Nanolithography is an emerging technique to create micro/nano features on substrate. New capillary based lithography method has been developed to overcome the limitations (e.g. directly contact of the substrate) of existing lithography techniques including dip-pen nanolithography nano-imprint lithography and electron-beam lithography. The understanding of the behavior of the liquid bridge formed between a capillary tube and a substrate is essential for the recently developed capillary based lithography method that is non-invasive to the substrate. A three-dimensional spectral boundary element method has been employed to describe the dynamics of liquid bridge. Starting with a steady-state liquid bridge shape, the transient bridge deformation is computed as the capillary tube is retracting away from the substrate. Several relations between the dynamic contact angle and contact line speed have been employed and discussed. The computational results are compared with experimental findings. The influences of liquid properties and retracting speed on the bridge dynamics are investigated.Copyright