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

Intrinsic electrochemical activity of single walled carbon nanotube–Nafion assemblies

The intrinsic electrochemical properties and activity of single walled carbon nanotube (SWNT) network electrodes modified by a drop-cast Nafion film have been determined using the one electron oxidation of ferrocene trimethyl ammonium (FcTMA+) as a model redox probe in the Nafion film. Facilitated by the very low transport coefficient of FcTMA+ in Nafion (apparent diffusion coefficient of 1.8 × 10−10 cm2 s−1), SWNTs in the 2-D network behave as individual elements, at short (practical) times, each with their own characteristic diffusion, independent of neighbouring sites, and the response is diagnostic of the proportion of SWNTs active in the composite. Data are analysed using candidate models for cases where: (i) electron transfer events only occur at discrete sites along the sidewall (with a defect density typical of chemical vapour deposition SWNTs); (ii) all of the SWNTs in a network are active. The first case predicts currents that are much smaller than seen experimentally, indicating that significant portions of SWNTs are active in the SWNT–Nafion composite. However, the predictions for a fully active SWNT result in higher currents than seen experimentally, indicating that a fraction of SWNTs are not connected and/or that not all SWNTs are wetted completely by the Nafion film to provide full access of the redox mediator to the SWNT surface

Fluid Dynamics of Material Micro-Deposition: Capillary-Based Droplet Deposition and Aerosol-Based Direct-Write

With rapid development of the direct-write technology, in addition to requirement of non-destructive printing, there is a need for non-expensive, robust, and simplified techniques of micro/nano fabrication. This dissertation proposes a new technique of non-invasive lithography called Capillary-Based Droplet Deposition and suggests improvements to existing Aerosol-Jet Direct-Write method that leads to deposition of thinner lines. A hollow capillary filled with liquid is a dispensing tool employed for the Capillary-Based Droplet Deposition method. Due to pressure applied from one side of the capillary, a liquid meniscus is formed at the opposite side of the capillary. After the meniscus touches the substrate, a liquid bridge between the capillary and substrate is formed. The capillary retraction causes the bridge rupturing and liquid droplet deposition. In the first part of this dissertation, the Capillary-Based Deposition method is considered both theoretically and experimentally. From bridge modeling, it is found that the droplet size is dependent on pressure applied, inner radius and wall thickness of the capillary, and liquid-capillary and liquid-substrate equilibrium contact angles. Three deposition scenarios are identified showing that minimum deposited droplet size is about 15% of the capillary inner diameter. Modeling results are verified in experiments with different water-glycerol solutions used as test liquid and with capillaries of wide range of inner diameters. The second part of the dissertation is devoted to theoretical investigation of the Aerosol-Jet Direct-Write method where few micron width lines are created from aerosol droplets that move along with the gas flowing through a converging micro-nozzle. Gas velocity and density profiles inside and outside of the nozzle are obtained from iv ANSYS/CFX simulation. Aerosol droplet trajectories and velocity components are calculated using all forces acting on the particles in the flow. Comparing all forces, it is found that only Stokes and Saffman forces are relevant for simulation of the gas-particle interaction. Original 1D equation for Saffman force is extended to two dimensional gas flows. For some parameter ranges, Saffman force is found to be negligibly small. Based on simulation results, two nozzle designs are proposed in order to collimate aerosol particles with diameters of 1.5-5.0 microns toward the nozzle centerline

MICRO COLD SPRAY DIRECT WRITE PROCESS

ABSTRACT 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