Reduction of thin oxide films on tungsten substrate with highly reactive cold hydrogen plasma

Results on chemical removal of oxygen from thin tungsten oxide films with hydrogen plasma are presented. Tungsten foils were oxidized in a furnace at 400 degrees C for 1 h, so a compact oxide film with the thickness of about 160 nm was formed. The samples were then exposed to hydrogen plasma created in a microwave discharge at the power of 1000 W and a hydrogen pressure of 90 Pa. The plasma density was estimated to the order of 10(16) m(-3), whereas the density of neutral hydrogen atoms was 2.5 x 10(21) m(-3) measured by a fiber optics catalytic probe. Samples were exposed to hydrogen plasma for different time. The evolution of the reduction process was monitored by measuring the AES depth profiles on samples exposed to plasma for different time. The results showed that the oxide film was reduced in about 10 s of plasma treatment. The average removal rate was thus about 16 nm s(-1). Such a high reduction rate cannot be explained by the interaction of hydrogen ions, because the ion flux onto the sample surface is orders of magnitude too low, but rather by the interaction of neutral H-atoms with the oxide film. Taking into account the known reduction rate and H-atom density, one can estimate the reaction probability to about 3 x 10(-4). Copyright (C) 2010 John Wiley & Sons, Ltd.