Simulation of PECVD
of SiO2 Film on Plastic Tubes
Da Zhang and Mark J. Kushner
Department of Electrical and Computer Engineering
University of Illinois at Urbana-Champaign
In medical applications most of the vials and tubes are made of glass and as a result can be easily broken. This results in a serious risk for health workers. Replacing glass tubes with plastic is not a simple matter since liquids can permeate through the tube wall. Since different components have different loss rates, the composition of the sample may change with time. A solution to this problem is to deposit a thin film of SiO2 on the the plastic wall as a permeation barrier. In this way we can preserve the advantage of using plastic materials by reducing permeation. In this project, we are investigating methods of depositing these coatings using Plasma Enhanced Chemical Vapor Deposition.
The economics of the PECVD process requires that many vials be simultaneously processed. For example, an array of vertically standing vials on the powered electrode of a PECVD reactor is one such possible arrangement(See schematic of the reactor). This arrangement makes the control of the fluxes and the uniformity of deposition more challenging.
TheHybrid Plasma Equipment Model (HPEM) has been used to investigate the uniformity of processing an array of vials. The HPEM simulates the particle density distribution in the plasma zone and derives particle fluxes and variations along tube surfaces. By running cases under different conditions, we can determine the influences on particle densities/fluxes resulting from reactor geometry, gas mixture, and substrate condition. We can therefore optimize the process for suitable deposition rate and film uniformity along the surfaces of the tubes.
We have found that the discharge properties are very sensitive to variations in the spacing of the vials and their locations in the reactor. For example, when changing the spacing between posts (post spacing) or that between the top of posts and the top wall of the reactor (top spacing), plasma density distributions and uniformity to the substrate change in systemmatic ways. (See Figures 1, , 2.). Optimum arrangements of post spacing and top spacing for achieving high uniformity of fluxes were derived (See Figures 3,, 4).
Hollow cathode behavior was observed for critical pd (pressure-distance) arrangements of the tubes, as was segmentation of the plasma into distinct regions, particularly with attaching gas mixtures (See Figure 5).
Work supported byBecton Dickinson, National Science Foundation, and the U of Wisconsin ERC.