Justin Hoey

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.

Solution-Based Synthesis of Crystalline Silicon from Liquid Silane through Laser and Chemical Annealing

We report a solution process for the synthesis of crystalline silicon from the liquid silane precursor cyclohexasilane (Si(6)H(12)). Polysilane films were crystallized through thermal and laser annealing, with plasma hydrogenation at atmospheric pressure generating further structural changes in the films. The evolution from amorphous to microcrystalline is characterized using scanning electron microscopy (SEM), atomic force microscopy (AFM), Raman spectroscopy and impedance spectroscopy. A four-decade enhancement in the electrical conductivity is attributed to a disorder-order transition in a bonded Si network. Our results demonstrate a potentially attractive approach that employs a solution process coupled with ambient postprocessing to produce crystalline silicon thin films.

Rapid Prototyping RFID Antennas Using Direct-Write

Optimization of radio-frequency identification (RFID) tags often requires several iterations of antenna design/fabrication/testing to meet cost and performance targets. The use of a rapid prototyping approach for antenna development would allow the designer an inexpensive and fast route to the refinement process. In this study, the performance of a commercial-off-the-shelf ultrahigh frequency (UHF) etched copper antenna was compared to printed silver antennas prepared by the following three direct-write techniques: maskless mesoscale materials deposition; matrix-assisted pulsed laser evaporation direct-write; and, collimated aerosol beam direct-write. The morphologies of the antennas were analyzed using contact and optical profilers with sheet resistance also being measured. Operational characteristics were determined by mounting silicon integrated circuits (IC) to the four different types of antennas. The performance of tags that utilized direct-write silver antennas was comparable to the copper-based commercial tag. To our knowledge, this is the first demonstration where some of the direct-write rapid prototyping attributes (e.g., slight overspray, overlap of written lines, overall thickness less than 500 nm) are shown to not seriously impede RFID tag performance. These results demonstrate the utility of direct-write for rapid prototyping studies for UHF RFID antennas.

Collimated Aerosol Beam Deposition: Sub-5-

Materials deposition based upon directed aerosol flow has the potential of finding application in the field of flexible electronics where a low-temperature route to printed transistors with high mobilities remains elusive. NDSU has been actively engaged in addressing this opportunity from the following two perspectives: 1) developing an appreciation of the basic physics that dominate aerosol beam deposition toward engineering a robust method that allows the realization of deposited features with sub-5 μm resolution; and, 2) developing an understanding of the mechanistic transformations of silane-based precursor inks toward the formation of electronic materials at atmospheric-pressure. In this paper, we will briefly discuss the genesis of a new materials deposition method termed collimated aerosol beam direct-write (CAB-DW) where precision linewidth control has been realized using a combined theoretical/experimental approach. Next, we will discuss progress using Si6H12 (cyclohexasilane-a liquid silane) as a precursor for solution-processed diodes and transistors. Finally, we demonstrate the ability to CAB-DW Si6H12-based precursor inks for printing Si-based semiconductors.

\mu

A study was performed to assess the viability of using ultra-high frequency (UHF) radio frequency identification (RFID) to communicate with subsurface sensors. The results showed that, when the reader operated near the FCC limit, read ranges for the sensorwere greater than a half meter for soil moisture levels of 15% by weight or less. Moisture levels at 15% or greater resulted in significant attenuation or complete loss of signal. Tags also had desireable read ranges for depths up to 15 cm.

m Resolution of Printed Actives and Passives

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

An investigation of the viability of UHF RFID for subsurface soil sensors

Cobalt silicide thin films were prepared by spin-coating liquid cyclohexasilane-based inks onto silicon substrates followed by a thermal treatment. The work function of the solution-processed Co–Si was determined by both capacitance–voltage (C–V) measurements of metal–oxide–semiconductor (MOS) structures as well as by ultraviolet photoemission spectroscopy (UPS). Variable frequency C–V of MOS structures with silicon oxide layers of variable thickness showed that solution-processed metal silicide films exhibit a work function of 4.36 eV with one Co–Si film on Si � 100 � giving a UPS-derived work function of 4.80 eV. Similar work function measurements were collected for vapor-deposited MOS capacitors where Al thin films were prepared according to standard class 100 cleanroom handling techniques. In both instances, the work function values established by the electrical measurements were lower than those measured by UPS and this difference appears to be a consequence of parasitic series resistance.

Micro Cold Spray Direct Write Process

Materials deposition based upon directed aerosol flow has the potential of finding application in the field of flexible electronics where a low-temperature route to printed transistors with high mobilities remains elusive. NDSU has been actively engaged in addressing this opportunity from the following two perspectives: (1) developing an appreciation of the basic physics that dominate aerosol beam deposition toward engineering a robust method that allows the realization of deposited features with sub-5 mum resolution; and, (2) developing an understanding of the mechanistic transformations of silane - based precursor inks toward the formation of electronic materials at atmospheric-pressatmospheric-pressureure. In this paper, we will briefly discuss the genesis of a new a materials deposition method termed collimated aerosol beam direct-write (CAB- DW) where precision linewidth control has been realized using a combined theoretical/experimental approach. Next, we will discuss progress using Si6H12 (cyclohexasilane - a liquid silane) as a precursor for solution-processed diodes and transistors. Finally, we demonstrate the ability to CAB- DW Si6H12-based precursor inks for printing Si-based semiconductors.

Work function characterization of solution-processed cobalt silicide

We present a detailed study of the laser crystallization of amorphous silicon thin films as a function of laser fluence and film thickness. Silicon films grown through plasma-enhanced chemical vapor deposition were subjected to a Q-switched, diode-pumped solid-state laser operating at 355 nm. The crystallinity, morphology, and optical and electronic properties of the films are characterized through transmission and reflectance spectroscopy, resistivity measurements, Raman spectroscopy, X-ray diffraction, atomic force microscopy, and optical and scanning-electron microscopy. Our results reveal a unique surface morphology that strongly couples to the electronic characteristics of the films, with a minimum laser fluence at which the film properties are optimized. A simple scaling model is used to relate film morphology to conductivity in the laser-processed films.

Collimated aerosol beam deposition: sub 5-μm resolution of printed actives and passives

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.