Chemical vapor deposition by pulsed ultrasonic direct injection of liquid precursors produces versatile method for creat
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INTRODUCTION The ultrasonic direct-injected, pulsed-CVD system manufactured by Sono-Tek is a robust, flexible and cost effective tool for producing prototype devices requiring thin films. Because of the unique vapor generation mechanism produced by using an ultrasonic nozzle, the system is very versatile. It can be used to make films using metalorganic compounds, polymers and nanophase suspensions. Unlike other CVD systems, the pulsed ultrasonic process uses no carrier gas or bubbler. The vapor is created by an ultrasonic nozzle and injected directly into the reaction chamber where a pressure "pulse" is produced by the evaporating droplets. The expanding vapor is driven down the cold wall reaction chamber (Figure 1) mixing and evaporating as it progresses. A 2.13” diameter monel susceptor plate positioned in the center of the quartz reactor causes molecular impingement of the vapor and allows surface adsorption to take place. Films as thick as several microns can be produced in minutes using this rapid prototyping method [1]. Several application specific, thin film prototypes: thermal barriers, stents, chemical sensors, and touch screens will be discussed in this paper.
Figure 1. The direct-injection ultrasonic-CVD process can be modeled using the Langmuir analysis of surface adsorption coupled with reaction kinetics when used for single component, heterogeneous, thermally activated reactions in a low-pressure reactor [1].
MATERIALS The ThinSonic pulsed ultrasonic CVD, manufactured by Sono-Tek Corporation of Milton NY1, incorporates a process developed by Cornell University. The pulsed ultrasonic method is a process in which a liquid precursor (metal, nanophase or macromolecule) is delivered to the ultrasonic nozzle through a series of automatically controlled solenoid valves. An adjustable ceramic piston pump determines the shot size and delivers this predetermined amount to a collector tube. After the programmed amount of precursor is accumulated in the tube, the solenoid valves change state. This state change introduces the precursor in a "controlled release" to the ultrasonic nozzle (Figure 2). The precursor is atomized at the tip of the nozzle and introduced into the low-pressure reaction chamber in a near-vapor-phase state. The mean diameters of the atomized drops are in the range of 15 microns (drop diameters are dependent on frequency, density and surface tension). The ultrasonic nozzle, simply stated, is a device that produces vibrations, which cause liquids to shear into small drops [2]. When a liquid film is formed by capillary action on the nozzle's atomizing surface and set into vibrating motion, some of the vibration energy is transformed into standing waves. These waves, known as capillary waves, form a rectangular grid pattern in the liquid on the surface of the nozzle. When the amplitude of the underlying vibration is increased, the amplitude of the waves increases correspondingly. Critical amplitude is ultimately reached at which the height of the capillary waves exceeds that required to mainta
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