Characteristics of C4F8 plasmas with Ar, Ne, and He additives for SiO2 etching in an inductively coupled plasma (ICP) reactor

Abstract

We have characterized the effect of adding Ar, Ne, and He noble gases to C4F8 inductively coupled plasmas for SiO2 etching. The systematic variation of their ionization potentials, metastable energy levels, and mass of the dominant ion in C4F8/X% discharges (X=He, Ne, or Ar) containing a high percentage of the noble gas provides a means to evaluate the relative importance of gas phase and surface processes in the etching of SiO2. The total ion flux, ion composition, FC deposition and etching rates, and composition of the surface reaction layer formed on Si or SiO2 in these discharges show systematic differences as a function of process parameters because of the different electron impact ionization thresholds for Ar, Ne, and He gases, and differences in the mass of the dominant ion for discharges containing a large proportion of the noble gas. For given experimental conditions (600 W and 20 mTorr for most of this work), Ar addition gives rise to the largest ion current density, and He to the smallest ion current density. When the noble gases are added to C4F8, the Ar+ ion flux contributes the largest percentage and He+ the smallest percentage of the total ion flux for the same dilution of C4F8 with the noble gases. Ion compositional analysis shows that CF+, CF3+, and CF2+ are the dominant ionic fluorocarbon (FC) species (in order of importance), and that they show similar trends as a function of added Ar, Ne, and He percentage. The fluxes of the more highly dissociated C+, F+, and SiFx+/COFx+ ions are greater when Ne and He are added to C4F8 than for Ar addition. Ion energy distributions of all ions are displaced to lower energies for C4F8/Ar discharges as compared with C4F8/Ne or C4F8/He plasmas. Infrared laser absorption spectroscopy was used to determine absolute densities of neutral CF, CF2, and COF2 radical species as a function of gas composition. The densities of CF2 and COF2 radical species were enhanced when Ne and He were added to C4F8 in comparison to Ar addition. Fluorocarbon deposition rates on unbiased Si substrates were measured and greater for C4F8 discharges with Ne or He additives than when Ar was added. Upon rf biasing of the substrates, the ion energies required for etching to take place are lowest for C4F8/Ar discharges, followed by C4F8/He and C4F8/Ne discharges. A comparison of x-ray photoelectron spectra of SiO2 surfaces exposed to C4F8/X% discharges at −10 V self-bias voltage shows pronounced differences in the chemical bond distribution of fluorocarbon films which can be explained by differences in momentum transfer to the surface and the associated bond breaking of deposited fluorocarbon layers on the surface. A comparison of SiO2 and Si etching rates in C4F8/X% discharges shows that for C4F8/Ne discharges containing more than 70%Ne the highest SiO2/Si but lowest SiO2/resist etching rate ratio is obtained.