A.J. Hydes, 'Spectroscopic Studies of the Etching of Si and SiO2 Substrates in CF4/O2 Discharges, PhD Thesis, 1985.

The investigation involved an extensive study relevant to silicon device technology of the optical emission and mass spectroscopic monitoring of an RF discharge of CF4/8%O2, an important system for the plasma etching of Si and SiO2.

Studies were performed on a commercially available single wafer plasma apparatus (PT440, ET Associates) using typical operating conditions of 180mTorr, 0.3W/cm2, discharge frequency 62.5kHz.

Optical emission spectra were recorded in the wavelength range 1900-7100Å (resolution ~ 0.3Å) resulting in the identification of F, F+, O, O+, C, CO, CO+, CO2+ and CF2. Si and Si+ were also observed when etching Si and SiO2 substrates. A spatially resolved optical study was undertaken in which a fine moveable light probe measured the intensity of the species F, F+, CO and CO2+ as a function of the position across the discharge. The optical work established that Si is predominantly etched by F atoms with a smaller contribution from metastable CO a3π (a triplet-PI). Results suggest that gas-surface reactions follow the spin conservation rule in the same way as they do for gas-phase reactions.

The mass spectroscopic sampling of gas from the discharge using a small quartz capillary in a combined static and spatially resolved study has shown that SiF4 is the major etch product detected from both Si and SiO2. Our collective results suggest however that SiF4 is formed by fluorination reactions of initially desorbed SiF and Si. Small concentrations of atomic Si and SiF3 radicals were detected. The results also suggest that CF2 and CF3 radicals are involved in the etching of SiO2.

XPS analysis of plasma etched Si and SiO2 substrates revealed high surface concentrations of F and C bonded species. The surface adsorbed species on Si and SiO2 substrates has been loosely characterised as SiFx (x~1) and SiFy (y~2.9) respectively. Highly over-etched Si substrates revealed a thick (~300Å) polymeric layer identified as (CF2)n, where n is large.

The adsorption characteristics of the etchant gases CF4, CHF3 and SiF4 on pure evaporated amorphous Si films at 273K were also investigated in a glass UHV apparatus by the volumetric method. The adsorption systems CHF3/Si and SiF4/Si both exhibited weak chemisorption obeying the molecular adsorption isotherm with surface coverages of ~0.9 and 0.4- 1 respectively. No measurable adsorption of CF4 by Si was detected.