Ion energy distribution functions in inductively coupled radio-frequency discharges—Mixtures of Cl2/BCl3/Ar

Abstract

We have used a gridded energy analyzer to measure positive ion energy distributions and fluxes at the grounded electrode of a high-density rf discharge. We present details of ion energies and fluxes in discharges containing mixtures of chlorine, boron trichloride and argon. These feedstock mixtures have been used extensively in the patterning of metal films for semiconductor interconnects. Our experiments were carried out in a Gaseous Electronics Conference Reference Cell which had been modified to produce inductively coupled discharges. The 15-cm-diam bottom electrode was grounded for these experiments. Stainless steel, anodized aluminum, and silicon were used as bottom electrode materials to understand the effects of various chamber boundary conditions on the ion energy parameters. In most cases, the ion energy distributions had a single peak, well separated from zero energy with a 1.0–3.0 eV full width at half maximum. These peaks were typically centered at 12–16 eV, with ion energy increasing as pressure decreased. Addition of BCl3 to a Cl2 discharge resulted in a reduction of total ion flux of more than a factor of 2, e.g., from 5 to 2.5 mA/cm2 (20 mTorr, 200 W). The ion fluxes decreased with increasing pressures in BCl3/Cl2 mixtures, while increasing with pressure in pure Cl2 discharges. Addition of Ar to several mixtures of BCl3/Cl2 changed the ion fluxes and mean ion energies by less than 15%. At high pressures or low inductive rf powers, the distributions can split into two overlapping peaks separated by as much as 8 eV. This splitting is indicative of capacitive coupling between the rf coil and the discharge. Insertion of a Faraday shield between the coil and the discharge removed most of the splitting.