Modeling and measurement of aerosol deposition on a heated substrate

Formation of an inorganic film by chemical aerosol deposition has been investigated experimentally and theoretically. Carrier gas flow rate, nozzle-to-substrate distance and substrate temperature were chosen as major process variables. The experimental work has been carried out to find their effect on the deposition efficiency, film thickness and its distribution. Both the deposition efficiency and film thickness increased with the carrier gas flow rate and substrate temperature but decreased with the nozzle-to-substrate distance. Especially at higher deposition rates, the central part of the film has a concave surface like a bowl. Flow and temperature fields of the fluid phase in the region between the nozzle and substrate were calculated numerically. Particle trajectories and particle evaporation were simulated numerically. As a result, the evaporation of the aerosol particles occurred so abruptly that the aerosol-existing region has a clear boundary. The extent of the region was found to be a determining factor in the film deposition, which characterizes the process of the chemical aerosol deposition.     

Preparation of nanostructured TiO2 thin films by aerosol flame deposition process

Titania thin films were prepared by using aerosol flame deposition process via the pyrolysis of titanium tetra-isopropoxide (TTIP) precursor. We analyzed the specific surface area, primary and secondary particle sizes, crystal structure, thin film morphology and thickness by BET method, electrophoretic light scattering, X-ray diffraction and scanning electron microscopy, respectively. The specific surface area of TiO₂ particles deposited is over three-times larger than that of commercial Degussa P25. Crystallite structure of TiO₂ particles can be controlled by changing the ratio of CH₄/O₂ flow rates. We could prepare TiO₂ thin films with single anatase phase by keeping the ratio of CH₄/O₂ flow rates at 200 ml/min: 1,000ml/min. As N₂ carrier gas flow rate to bubbler increases, the primary and secondary particle sizes increase, but decrease with increasing total N₂ gas flow rate through the central tube. The shorter the deposition height is, the smaller the deposition area is, but the thin film becomes thicker in the central region.     

Exhaust Aerosol of a Plasma Enhanced CVD System: II. Electrical Charging and Transport

The microelectronics industry has been concerned about the loss of product yield in its semiconductor wafer processing steps from killer defects caused by the presence and deposition of contaminant particles. Reactant gases used in plasma enhanced chemical vapor deposition (PECVD) can form nanometer sized particles from homogeneous nucleation. Once the particles grow to a few nanometers, they become negatively charged due to the collected ion and electron currents on the particle surface. A gradient of the electron and ion concentration between the sheath and bulk plasma causes an electric field to develop directed to the walls. Contaminant particles can eventually become trapped in electrostatic potential wells due to the higher charge density of positive ions near the powered electrode. The traps fill in the plasma sheath region until some particles are "leaked out" by gas drag forces. Therefore particles formed solely in the plasma volume are theorized to possess a distinct charge level from condensation particles originating within the exhaust line alone. Consequently, plasma properties determine the size and charge distribution of contaminants that exit the reactor and enter the exhaust line. The intended contribution of this research was to (1) develop the capability to monitor charged and uncharged contaminant particle fractions during a thin film deposition cycle and (2) to understand the coupled effect of particle transport and charging in an radio frequency (RF) plasma. Therefore the size distribution and absolute electrical charge were measured in the vacuum exhaust line during a SiO 2 thin film cycle using an integral mobility charge analyzer. Experiments confirm that over 50% of fine particles in the exhaust were neutral. However, a moderately charged coarse fraction was measured between 0.15 and 0.3 w m. The larger particles are hypothesized to be reactor particles gradually released from the potential wells in the plasma sheath. Process variables, including the RF power and the reactor pressure, directly affected the size and charge characteristics measured on particles in the exhaust line. The first paper in the series "Exhaust Aerosol of a Plasma Enhanced CVD System" uses an in-line exhaust sampling system to measure the size distribution of contaminant particles exiting the plasma.     

Deposition of Charged Aerosol Particles on a Substrate by Collimating Through an Electric Field Assisted Coaxial Flow Nozzle

Electroaerodynamic (EAD) jet printing, where aerodynamic force is coupled with electrostatic force in order to obtain a wide range of controlled pattern sizes, is introduced. Charged and sheathed aerosol particles yield a high deposition rate even at low velocity owing to the force of their electrostatic attraction to the substrate. In this study, two coaxial nozzles (inner diameters of 6 mm and 100 μm) were designed and tested theoretically and experimentally in order to observe the effects of electrostatic force, particle size, and air flow rate on particle trajectory and dot pattern size. A higher sheath air flow rate (higher Stokes number) caused the aerosol jet stream to be focused. For Stokes numbers higher than 1, the effect of applied voltage on pattern size was less than that of the sheath air flow rate. However, for Stokes number lower than 1, the pattern size was affected by both the applied voltage and the sheath air flow rate. After incorporating all data, the diameter of the particle deposition area (W_p) was expressed as a function of nozzle diameter (W), sheath air flow rate (Q_sheath), aerosol flow rate (Q_aerosol), Stokes number (Stk), and Electrostatic number (Es). Three different equations were obtained for Stk < 1, for 1 ≤ Stk < 5, and for Stk ≥ 5, respectively. These equations would be used to predict pattern width for given conditions of aerosol and sheath flow rates, particle size, electric field, and nozzle size.

Copyright 2013 American Association for Aerosol Research     

Enhancement of Extrinsic Charging Efficiency of a Nanoparticle Charger with Multiple Discharging Wires

A unipolar charger with multiple discharging wires has been developed and investigated to enhance the extrinsic charging efficiency of nanoparticles by using sheath air near the wall of the charger. The applied voltage of the charger ranged from +4.0 to +10 kV, corresponding to corona current from 0.02 to 119.63 μA. Monodisperse NaCl particles of 10 ～ 50 nm and Ag particles of 2.5 ～ 10 nm in diameter were produced to test the performance of the charger with multiple discharging wires and to investigate the particle loss at different sheath flow rates, corona voltages and sheath air velocities. Results showed that the optimal efficiency in the charger was obtained at +9 kV applied voltage, 10 L/min aerosol flow rate and 20 L/min sheath air flow rate. The extrinsic charging efficiency increased from 2.86% to 86.3% in the charger as the particle diameter increasing from 2.5 to 50 nm. The TDMA (tandem-differential mobility analyzer) technique was used to investigate the charge distribution, and the charge distributions in the exit were obtained at the optimal operating condition.     

Simulation of silicon film growth by silane decomposition at low pressures and temperatures

Silicon film deposition by silane decomposition in LPCVD (Low Pressure Chemical Vapor Deposition) process has been simulated by numerical computation of the governing transport and reaction equations, assuming that the rate of silane decomposition in the gas phase controls the overall film growth rate. The film growth rate and the film uniformity increase with the reactant flow rate when the flow rate is relatively low, but they decrease at higher flow rates due to the negative effect of the reduced reaction time in the reactor. Accordingly, the film deposition process is optimized by controlling the reactant flow rate so that the position of the maximum SiH₄-decomposition rate in the gas phase is located above the substrate region. With a larger degree of the substrate tilting, the growth rate decreases but the film uniformity is improved. The film uniformity is also improved when the pressure is low.     

Numerical Modeling of Particle Dynamics in a Cylindrical Chamber Containing a Rotating Disk

Numerical modeling was performed to study the submicron particle dynamics in a confined flow field containing a rotating disk, temperature gradient, and various inlet gas flow rates. The Lagrangian model was employed to compute particle trajectories under the temperature gradient, disk rotation speed, and inlet gas flow rate effects. The trajectories of particles with diameters of 1 μm, 0.1 μm, and 0.01 μm were examined in this study. When the inlet gas temperature was lower than that of the disk, particle-free zones were created due to upward thermophoretic force for 1 μm and 0.1 μm particles. Disk rotation was found to depress the size of the particle-free zone. Particle deposition onto the disk for 0.01 μm particles was possible because of the Brownian motion effect. A detailed evaluation of the particle-free zone size as a function of the temperature gradient, disk rotation speed, and inlet gas flow rate was performed. When the inlet gas temperature was higher than the disk temperature, particle deposition onto the disk was enhanced due to the downward thermophoretic force for 1 μm and 0.1 μm particles. Disk rotation was found to increase the deposition rate. For 0.01 μm particles, Brownian motion was more important than thermophoretic force in controlling particle behavior. The particle deposition rates as a function of the temperature gradient, disk rotation speed, and inlet gas flow rate were performed.     

Exhaust Aerosol of a Plasma Enhanced CVD System: I. Size Distribution Measurements

Understanding the origin and fate of plasma-enhanced chemical vapor deposition (CVD) contaminant particles is a critical issue in semiconductor manufacturing in order to improve thin film deposition on wafer surfaces. Several competing external forces will affect a particle's motion in the plasma field prior to either landing on the wafer or entering the exhaust line. Electrical forces dominate during plasma radio frequency (RF) activation creating regions of potential wells. If trapped, the nucleated particles can continue to grow and gain electron charges until gas or ion drag forces can overcome the potential barrier. Mutual electrostatic repulsion between particles can also cause the traps to "leak" out contaminants into the exhaust line. In this way, contaminants formed solely in the plasma volume are hypothesized to possess a distinctive size and charge distribution independent of condensation particles originating from gas compression by the oil-based rotary pump. For these reasons, a novel experimental aerosol sampling system was designed to continuously monitor submicron particles carried during a thin film deposition cycle without disturbing the internal operation of the plasma. Sampling from the plasma enhanced chemical vapor deposition (PECVD) process exhaust gases using an oil-free mechanical piston pump parallel to the main vacuum line is considered to be an effective alternative to in situ probe measurement. Concentration and size distribution data were continuously measured using a condensation nucleus counter and an optical spectrometer. Results show that the particles in the reactor exhaust line are bimodal or made up of fine and coarse sizes divided near 100 nm. Experimental results show the fine fraction increases in the exhaust line after a certain time interval. This delay is hypothesized to be the initial period the nucleated particles were trapped inside the plasma's potential wells. Once trapped, reactor particles can continue to grow in agreement with free molecular coagulation models. A larger particle will experience greater gas drag to eventually overcome the electrical forces. The delay, or critical transport time, depends upon the reactor pressure and plasma power, which also affect the size of the trapping field. The second paper in the series "Exhaust Aerosol of a Plasma Enhanced CVD System" compares a computational charging model of the plasma sheath with experimental charge distribution measurements of contaminant particles carried through the CVD exhaust.     

Study of a Nanoparticle Charger Containing Multiple Discharging Wires in a Tube

Abstract

A unipolar charger containing multiple discharging wires in a tube (inner diameter: 50 mm) was developed and tested in order to increase the aerosol flow rate and the charging efficiency of nanoparticles. Four gold wires of 25 µm in diameter and 15 mm in length were used as the discharging electrodes to generate positive ions (N_i) from 2.72 × 10⁸ ions/cc to 3.87 × 10⁹ ions/cc in concentration at the discharging voltage of + 4.0 ～ + 10 KV. Monodisperse NaCl particles of 10 ～ 50 nm in diameter were used to test the charging efficiency and the particle loss of charged particles with different aerosol flow rates, corona voltages and sheath flow rates. The sheath air near the tube wall was found to increase the extrinsic charging efficiency, and the highest efficiency was obtained at + 6.0 KV discharging voltage, 10 L/min aerosol flow rate and 9 L/min sheath flow rate. The extrinsic charging efficiency increased from 10.6% to 74.2% when the particle diameter was increased from 10 to 50 nm. The TDMA (tandem differential mobility analyzer) method was used to determine the charge distribution and the mean charge per particle and it was found that the Fuchs charging theory corrected for the extrinsic charging efficiency matched with the experimental data very well.     

Microparticle Transport and Deposition in the Human Oral Airway: Toward the Smart Spacer

A key issue in pulmonary drug delivery is improving the medical delivery device for effective and targeted treatment. Spacers are clear plastic containers attached to inhalers aimed at delivering more drug particles to the respiratory tract. The spacer's one-way valve plays an important role in controlling and initializing the particles into the oral cavity. This article studied particle inhalation and deposition in an idealized oral airway geometry to better optimize the spacer one-way valve shape and design. Three steady flow rates were used 15, 30, and 60 l/min and a Lagrangian, one-way coupling particle tracking model with near-wall turbulence fluctuation correction was used to determine the deposition rates. For all three breathing rates, the velocity field in the midsagittal plane showed similar gross fluid dynamics characteristics, such as the separation and recirculation regions that occur after the larynx. The particle deposition rates compared reasonably well with available experiments. Most particles deposited at the larynx, where the airway has a decreasing cross-sectional area. For different particles sizes, most particles introduced at the lower region of the mouth show higher possibility to pass through upper airway and enter the trachea and lung airways. The particle deposition patterns in the airway were traced back to their initial inlet position at the oral inlet; and this information provides the background for a conceptual and optimized design of the spacer one-way valve.

Copyright 2015 American Association for Aerosol Research     

流态化气相沉积过程中稀释气体流量对颗粒表面SiO2沉积的影响

利用Fe(Si)合金球形粉末为沉积基底,正硅酸乙酯为SiO2气相介质前驱体,采用引入流化环节的化学气相沉积工艺合成了 Fe(Si)/SiO2复合粉末.考察了沉积过程中Ar稀释气体流量对Fe(Si)基底粉末表面SiO2绝缘介质沉积过程的影响规律及形成完整核壳异质结构的稀释气体流量范围.实验结果表明,随着流态化气相沉积过程...     

Charged Particle Trajectory Statistics and Deposition in a Turbulent Channel Flow

The statistical properties of charged particles and their wall deposition in a turbulent channel flow in the presence of an electrostatic field is studied in this paper. For a dilute concentration, the influence of small particles on the fluid motion is neglected. The instantaneous velocity field is generated by a direct numerical simulation of the Navier-Stokes equation via a pseudospectral method. The case in which each particle carries a single unit of charge and the case in which the particles have a saturation charge distribution are analyzed. Ensembles of 8192 particle trajectories are used for evaluating various statistics. Effects of size and electric field intensity on particle trajectory statistics and wall deposition rate are studied. RMS particle velocities and particle concentrations at different distances from the wall are evaluated and discussed. The results for deposition rates are compared with those obtained from empirical equations.     

Atomic bonds in boron carbon nitride films synthesized by remote plasma-assisted chemical vapor deposition

Boron carbon nitride (BCN) films are synthesized by remote plasma-assisted chemical vapor deposition (RPCVD) method. The present experimental apparatus is featured by introducing BCl₃ gas near the substrate without mixing to plasma consisting of N₂ and CH₄ gases. Two sample groups of the BCN films are prepared. One is grown with various CH₄ flow rates, and another is grown with various BCl₃ flow rates. The composition ratio of the constituent atoms, atomic bonds and optical bandgap are investigated. C composition ratio of the BCN film increases with increasing CH₄ flow rate, leading to a reduction in the optical bandgap with increasing C composition ratio. On the other hand, it is found that no significant variation in the composition ratio occurs for the BCN films grown with various BCl₃ flow rates and that the optical bandgap decreases with increasing BCl₃ flow rate. This behavior of the optical bandgap is related to a change of the atomic bonds in the BCN film grown with various BCl₃ flow rates.     

Deposition of magnetite particles from flowing suspensions under flow‐boiling and single‐phase forced‐convective heat transfer

The deposition rate of colloidal magnetite particles was measured under both single‐phase forced‐convective and flow‐boiling conditions. All measurements were made at alkaline pH where both the heat transfer surface and the surface of the magnetite particles appear to be negatively charged. For single‐phase forced convection, the deposition rate constant is lower than the mass transfer coefficient for colloidal particles, and the difference is attributed to the force of repulsion between the negatively charged surfaces of the particle and substrate. The deposition rate measured under flow‐boiling conditions is lower than that reported for the deposition of colloidal particles at neutral pH. The difference is, again, attributed to the force of repulsion between the particle and substrate. Particle removal rates were significantly lower than deposition rates; analysis using the theory of turbulent bursts suggests a removal efficiency of only 10^?9% for each turbulent burst. The low removal efficiency is consistent with the particle diameter being significantly smaller than the thickness of the laminar sublayer in these tests.     

Effects of gas temperature fluctuation on the instantaneous char reaction of pulverized coal particle

The heterogeneous char reaction processes of pulverized coal particle in a hot gas flow with temperature fluctuation are investigated. The instantaneous mass variations and char reaction rates of the particles with initial diameters of 10-50 μm are calculated under different conditions. The gas temperature fluctuation has evident influences on the instantaneous char reaction processes of the pulverized coal particles. The instantaneous char reaction rates with the gas temperature fluctuation are different from those without the gas temperature fluctuation. The gas temperature fluctuation leads to more rapid char reaction and faster mass loss of the particles. The effects of fluctuation amplitude of the gas temperature and particle Reynolds number on the instantaneous char reaction processes are delineated.     

Particle Transport and Deposition in a Hot-Gas Cleanup Pilot Plant

ABSTRACT

Development of hot-gas filtration systems for advanced clean coal technologies has attracted considerable attention in recent years. The Integrated Gasification and Cleanup Facility (IGCF), which is an experimental pilot plant for testing performance of ceramic candle filters for hot-gas cleaning, has been operational at the Federal Energy Technology Center (FETC) in Morgantown, West Virginia, for several years. The present work describes a computer simulation study of gas flow and particle transport and deposition in the IGCF filter vessel with four filters. The stress transport model of FLUENT? code is used for evaluating the gas mean velocity and the root mean-square fluctuation velocity fields in the IGCF filter vessel. The instantaneous fluctuation velocity vector field is simulated by a filtered Gaussian white-noise model. Ensembles of particle trajectories are evaluated using the recently developed PARTICLE code. The model equations of the code include the effects of lift and Brownian motion in addition to gravity. The particle deposition patterns on the ceramic filters are evaluated, and the effect of particle size is studied. The results show that, for a clean filter (just after the backpulse), the initial deposition rate of particles on the candle filters is highly nonuniform. Furthermore, particles of different sizes have somewhat different deposition patterns, which could lead to nonuniform cake compositions and thicknesses along the candle filters. The effects of variations in the filter permeability on the vessel gas flow patterns and the pressure drop, as well as on particle transport patterns, are also studied.     

On the leakage flow around gas bubbles in slug flow in a microchannel

The leakage flow is that liquid does not push gas bubbles and leaks through the channel corners. This leakage flow was confirmed by tracking particles moving in the liquid film with a double light path method and was quantified by tracking the gas–liquid interface movement. The results show that leakage flow varies during bubble formation process. The average net leakage flow Q_net‐leak in a bubble formation cycle at T‐junction can be as large as 62.4% of the feeding liquid flow rate, depending on the liquid properties. Q_net‐leak for regular flow at main channel is much smaller, ranging from about 0 to 30% of the feeding liquid flow rate. The difference between the two leakage flows would lead to an increase in liquid slug length after generation. Finally, the effects of parameters such as phase flow rates, surface tension, and viscosity were investigated. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3964–3972, 2015     

Numerical analysis on premixed combustion of H2–SiCl4–Air system to prepare SiO2 particles

In this study, we used the commercial CFD-code FLUENT to analyze numerically the hydrogen combustion and SiO₂ particle formation in the premixed flame reactor. We considered SiCl₄ as a precursor for SiO₂ particle formation and calculated the profiles of fluid flow, temperature, species concentration and reaction rates for 2D premixed reactor model in FLUENT. Using the data for temperature and velocity extracted from FLUENT, we calculated the trajectories and temperature histories of SiO₂ particles moving inside the premixed flame reactor and those particles starting near the centerline of reactor pass through the maximum temperature histories.     

错流移动床的压降特性

在矩形移动床内考察了颗粒下移速度、颗粒堆积状态及空腔生成和长大发展过程等因素对压降的影响. 在错流气体速度为0.09~1.35 m/s、颗粒下移速度为0.95~9.68 cm/min的较大变化范围内进行了实验研究. 结果表明,颗粒下移速度对压降几乎没有影响;当错流气速足够大时移动床内将出现"空腔"和"贴壁"等现象,空腔的发展过程造成压降随时间出现稳定、微波脉动和大幅波动3种变化;欧根公式适用于低错流气速时的移动床压降;高错流气速下空腔出现了"生成-长大-塌落-流化"的循环变化过程. 在实验基础上建立了有空腔时的移动床压降模型,并对空腔尺寸进行了无因次关联,其床层压降的计算结果与实验值相符.     

Preparation and characterization of TiO2 thin films by PECVD on Si substrate

Titanium oxide thin films were prepared on p-Si(l00) substrate by plasma enhanced chemical vapor deposition using high purity titanium isopropoxide and oxygen. The deposition rate was little affected by oxygen flow rate, but significantly affected by RF power, substrate temperature, carrier gas flow rate, and chamber pressure. Morphology of the film became coarser with increasing deposition time and chamber pressure, and the film showed less uniformity at high deposition rates. It was also found that the overall deposition process is controlled by heterogeneous surface reaction below 200°C., but controlled by mass transfer of reactants at higher temperatures. TiO₂ films deposited at temperatures lower than 400°C was amorphous, but showed the anatase crystalline structure upon 400°C deposition. The dielectric constant was about 47 for the films post-treated by rapid-thermal annealing (RTA) at 800°C. The leakage current was about 2×10⁻⁵ A/cm² for the films deposited at 400°C and RTA-treated at 600°C. However, it was decreased to less than 3×10⁻⁷ A/cm² for the film RTA-treated at 800°C.