ICMCTF2010 Session G3: Atmospheric and Hybrid Plasma Technologies

Resorting to atmospheric pressure plasmas (APP) in the field of PECVD offers several interests. Because of their versatility, APP have pratical application in surface cleaning, deposition of thin films or coating of nanoparticles, etc. They could also be used to create metamaterials. This quality is mainly due to the large availability of sources operating at atmospheric pressure. Various plasma sources have been proposed, including DC arc, corona discharges, Dielectric Barrier Discharges, or microwaves excited plasmas.

The recent development of small-scale plasmas, including micro-plasmas, offers new capabilities. The APP may be used to localize the surface treatment on very small areas. Several new applications emerged from this capability. For example, one can pattern a surface with stripes of different wettabilities for lab-on-chip application and micro-chemical analysis systems. Bio-MEM sterilization, displays and micropropulsion are also possible fields of application. About PECVD, localized treatments could be useful to structure surfaces or to process small-scale materials. This idea was first proposed by Babayan et al.[1] . They used an atmospheric-pressure plasma jet that was produced by flowing oxygen and helium between two coaxial metal electrodes driven by 13.56 MHz radio frequency power. The remote plasma exiting from between the electrodes was mixed with tetraethoxysilane. Since these pioneer works, other developments were proposed. The “Torche à Injection Axiale”, also called the TIA was used to perform such localised PECVD treatments at atmospheric pressure but at much higher temperatures than those encountered in DBDs. Another interesting approach deals with microstrip split-ring resonator microplasma sources. With such a device, self-ignition in atmospheric air could be accomplished with a device that has a gap size of only 25μm.

Recently, we have developed a small-scale remote plasma at atmospheric pressure based on a resonant microwave cavity. By controlling the hydrodynamics of the gas, we have shown that it is possible to extract through a tinny hole in the cavity a straight beam of active species over a length of 10 cm [2]. We will describe a way to solve the scaling-down of the sources to reach the nanoscale.

[1] S E Babayan, J Y Jeong, A Schütze1, V J Tu, M. Moravej, G S Selwyn and R F Hicks, Plasma Sources Sci. Technol. 10 (2001) 573–578

Hollow cathode plasma (HCP) has been used for sputtering apparatus because of its localized high density characteristics. An array of HCP’s has once been used to generate large area plasma for chemical vapor deposition of amorphous silicon in TFT-LCD manufacturing, where applied RF power is usually small.

In this work, array of HCP’s has been designed to have high density hydrogen plasma for photoresist etching process, where high RF power is needed. Twenty hollow cavities of cylindrical shape were formed in an aluminum cathode plate of 200mm diameter. After engraving hollow cavities, the cathode plate was anodized. In general, plasma generation from the hollow cavity depended on cavity size, chamber pressure, process gas used and applied RF power.

After diameter and depth of the cavity were varied, we found cavity with 6 mm in diameter and 10 mm in depth showed the highest density in the pressure of 2 Torr with 2.5 KW of RF power in case of hydrogen. Arc discharge could occur at the inside and outside corners of the cavities and sputtering phenomena in the cavity was observed after a long time operation. Redistribution of injection holes of process gas in the cathode plate and insertion of ceramic tub with rounded shape into the cavities could successfully solve these problems.

For photoresist stripping a baffle with holes was placed between the cathode and the wafer stage in order to have hydrogen radicals for the stripping reaction. Stripping rate of 200 nm/min and uniformity of 7 % could be obtained over 300 mm wafers. This high density hydrogen plasma can be a very effective method of photoresist stripping in dual damascene process of copper metal and low-k dielectrics where oxygen plasma cannot be used.

This work presents the synthesis of polypyrrole nanoparticles by radiofrequency capacitive atmospheric pressure plasmas. The polymerization was done in a tubular glass reactor with stainless steel caps and square electrodes with a gap of 2 mm . Each electrode was covered with glass to form a dielectric barrier which homogenizes the plasma discharges. The characterization of the polypyrrole particles was done by thermal analysis; IR and fluorescence spectroscopy; and by scanning, transmission, and atomic force electronic microscopy.

The results showed that the polymers were formed in spherical conformation with core-shell morphology. Some of the particles joined to form thin films and others grew as separated spheres with external diameters from 130 to 280 nm. The wall thickness in the bigger particles reached up to 100 nm. The core of these particles is filled with agglomerates of several much smaller particles with individual diameter in the order of 40 nm. These polypyrrole nanoparticles have similar chemical structure as other films prepared with plasmas of pyrrole. Both, films and nanoparticles, can be combined to increase and enhance the applications of this polymer in the area of medicine and semiconductors.

High performance synthetic fibers, such as ultra-high molecular weight polyethylene (UHMW-PE), are key components in achieving energy absorption and/or toughening properties in fiber-reinforced polymer composites. However, these fibers exhibit poor wettability and adhesion when incorporated into common resin matrix systems, thus resulting in poor mechanical performance. To evade this problem, many have utilized various modes of surface modification techniques, the more common ones being the use of chemical coupling agents, acid treatments, and plasma treatments. The advantages of atmospheric pressure plasma surface treatments such as quick processing times, ability to treat large areas, and the effectiveness in creating chemical species outweigh those of other treatments.

We report the use of atmospheric pressure dielectric barrier discharge (DBD) plasma surface treatments to functionalize the surfaces of ultra-high molecular weight polyethylene (UHMW-PE) fibers and its effect on the chemical composition at the surface of these fibers and bulk material properties. Different plasma treatment conditions consisting of various exposure time and gas flow-rates were implemented to determine observable trends in the formation of specific chemical groups. X-ray photoelectron spectroscopy (XPS) coupled with conventional titration techniques were used to quantify the surface concentration of reactive groups, whereas the surface roughness before and after plasma treatment was determined by atomic force microscopy (AFM). To characterize the plasma and to elucidate plasma-surface interactions that occur within the plasma optical emission spectroscopy (OES) studies were employed. The correlation between surface group concentrations and surface morphology/roughness will be discussed.