Mass Spectral Investigation of the Radio-Frequency Plasma Deposition of Hexamethyldisiloxane

Abstract

Experiments have been performed with the aim of enhancing our understanding of the reactions that take place within inductively coupled, radio-frequency (RF) plasmas of hexamethyldisiloxane (HMDSO). These plasmas have been investigated using a combination of mass spectrometry (MS) and deposition rate measurements. Thin films of HMDSO were deposited onto silicon substrates and analyzed by X-ray photoelectron spectroscopy (XPS). At low plasma power, the positive-ion MS data reveal that extensive molecular oligomerization takes place within the plasma gas phase. However, no significant formation of neutral oligomeric species was detected. XPS analysis revealed the surface stoichiometry of the deposit to be closer to that of the oligomers detected by MS than that of the starting compound (M). At higher plasma power, increased fragmentation of the monomer was observed, with a concomitant loss of the higher mass positively charged species. Again, no significant formation of neutral species of mass greater than M was detected. The deposition rate increased from ca. 6.5 to 15 ng s-1 over the power range investigated. We conclude that at low plasma power ion-molecule reactions are responsible for deposit formation. We propose that at high power the ionic component of these plasmas is still important, in terms of deposition.