Paper PS2+BI-ThA9
Correlation of Properties of Polymeric Organic Layers with Plasma Parameters

Thursday, October 21, 2010, 4:40 pm, Room Galisteo

For exact deposition of thin films out of the vaporous phase (cvd), an entire knowledge of the process parameters such as flows, pressure and gaseous temperature is required. In the case of pecvd, this means the extension on influencing plasma variables like plasma density and electron temperature, in particular in large reactors for production purposes to meet the demands for flat layer qualities (growth and composition) over the whole reactor volume.

In an almost cubical reactor 80 l in volume, the microwave power is coupled into the volume via a quartz window which exhibits approximately 1/10 of the sidewall area. The spatial compilation of these plasma quantities along with plasma potential has been accomplished with a bendable Lang­muir probe. To isolate the tungsten wire against its grounded housing tube, it was coated with poly­parylene. After having compared this construction with our Lang­muir probe which has been now in use for more than a decade, we have taken data of the whole reactor with argon and with mixtures of monomers of parylene and argon or oxygen in a pressure range between 10 mTorr and 150 mTorr (1 1/2 Pa to 20 Pa) applying a new evaluation procedure [1]. Over the covered range, the plasma density remains in the dielectric regime (plasma degree less than 100 ppm).

Compared to discharges through pure argon, the plasma parameters exhibit opposing be­havior: at same discharge pressure and power input, the plasma density is lower, whereas the electron temperature goes up. The layers are highly transparent with a slightly yellow color. Ftir measurements reveal that the ring structure still remains intact. Adding oxygen to the ambient to the monomeric vapor leads to hydro­philic surfaces which is caused by the formation of CO bonds and OH bonds. The creation of these features is confined by power input. If it is raised beyond 4 W/l, the reaction mechanism drastically changes from surface polymerization to volume poly­merization lead­ing to thick, low-density films which can be easily be scratched away. This change has been traced by plasma diagnostics and mass spectrometry. At a threshold density of about 1 x 10¹⁰cm³ (plasma degree about 1000 ppm), all peaks beyond 44 (CO₂) vanish. In the resulting mass spectrum, no CH vibrations beyond 3000 cm^-1 can be detected indicating the complete destruction of the aromatic system.

[1] Peter Scheubert: Modelling and Diagnostics of Low Pressure Plasma Discharges, PhD thesis, Bochum, 2002