Influence of the discharge current on the deposition of HMDSO/O2 - plasma at polypropylene membranes

Abstract

In this report the influence of the discharge current on the deposition of HMDSO/O2 onto polypropylene (PP) membranes is discussed. The experiments took place at a low frequency (3.8 kHz) in a cylindrical glass reactor. The gas mixture in the reactor consisted of 90% O2 and 10% HMDSO reacted at a low pressure of 40 mTorr. The polypropylene substrate was positioned on the lower electrode, and was covered with a layer of 100 nm of plasma deposited film. Several different diagnostic techniques were used to study the influence of the discharge current ld on the permeability of the treated PP membranes such as thickness and density measurements, FTIR absorption spectrometry and of course permeability measurements. When the discharge current increases, that is to say the discharge power Pd increases, the autopolarisation voltage VDC increases (in absolute value) for currents larger than 1 mA as does the deposition rate, Vdep. The inorganic part of the film R remained very low under the process circumstances used, which points to a large quantity of organic bonds (Si-(CH3)3, Si(CH3)2, C=O....) in the film. With increasing plasma power, the flux of oxygen through the membrane decreases while the density, the deposition rate and the R-factor increase. The quality of the plasma deposited membrane improves. It is difficult to create a homogeneous layer, because of the formation of pleats during deposition, due to ionic bombardments. The goal of creating a uniform thickness of 100 nm on the PP substrate is difficult to reach, because of the very short manipulation time (of 30 seconds) available. During this time the desired current has to be set and several electrical quantities must be measured. The uniformity of measurements is also affected by variations in gas mixture, furthermore a difference in calculated deposition rate occurs. The plasma polymer film with the best results, regarding 02 permeability, is reached at the highest currents (here 8 mA); the maximum reduction in flow, when compared to non-deposited membranes, is 17 %.