MASS-SPECTROMETRIC STUDY OF TETRAETHOXYSILANE AND TETRAETHOXYSILANE OXYGEN PLASMAS IN A DIODE TYPE RADIOFREQUENCY REACTOR

Mass spectrometry of tetraethoxysilane (TEOS) vapor, TEOS, and TEOS-O2 radio-frequency plasmas has been investigated in a diode type plasma reactor. The TEOS vapor spectrum shows eight main heavy ions containing silicon and oxygen in a background of many second order ions. When coupling a pure TEOS plasma (13.56 MHz, 50 W, 150 mTorr) the same ions are observed but with variable abundance. For low m/e range (1-50) the variations show that hydrocarbons and alcohols formation is induced by the discharge. When adding a second reactant like oxygen the TEOS dissociation increases. Four ions in the low m/e range show a sensitive variation in the quadrupole mass spectra (QMS) (CO+,CO2+,COH+,COH2+). This corresponds to the formation of stable volatile compounds containing oxygen (CO,CO2,COH2) in the gas phase and on the surface: the oxidation occurring on the surface is associated with a desorption effect which removes carbon atoms from the growing film on the electrodes and walls. The QMS results are related to Fourier transform infrared (FTIR) results of deposited layers on silicon substrate placed on the grounded electrode: for a pure TEOS plasma or a TEOS-He plasma the undecomposed TEOS molecules recondense on the growing film giving a nonadherent gel-like layer when operating at low substrate temperature; characteristic vibration modes of the TEOS molecules (C-H, C=O, Si-O-C) appear in the FTIR spectrum. The intensity of the corresponding peaks decreases to zero when gradually introducing oxygen: the increase of the TEOS dissociation limits the TEOS recondensation on the film; a correlation is established between the surface oxidation and CO, CO2, COH2 desorption and the disappearance of the infrared carbon vibration modes.