Fluorocarbon high-density plasmas. I. Fluorocarbon film deposition and etching using CF₄ and CHF₃

Abstract

Fluorocarbon film deposition in discharges used for oxide etching plays a key role in determining the profile shape of contact holes and the etch selectivity with respect to the mask and the underlayer. For low-density capacitatively coupled rf discharges this deposition is due to neutral radicals. We report a study of fluorocarbon film deposition and etching phenomena in electron cyclotron resonance (ECR) discharges of CF4 and CHF3. Plasma operation without rf sample bias in the pressure range below 10 mTorr results in the deposition of fluorocarbon films for both gases, with the highest deposition rate in each case at 2 mTorr (=120 nm/min for a 1000 W CF4 plasma and =180 nm/min for CHF3 using the same conditions). For CF4 this behavior differs dramatically from that seen for conventional rf diode plasmas where no deposition occurs. The deposition is due to the more efficient breakdown and ionization of CF4 and CHF3 in the ECR discharge and the lack of energetic ion bombardment of the substrate as compared to capacitatively coupled rf diode plasmas. We have used a double grid ion energy analyzer in front of a silicon wafer being ellipso-metrically sampled to unambiguously demonstrate that in these high-density discharges, fluorocarbon deposition is primarily due to bombardment with low energy ions. The fluorocarbon growth rate dropped by a factor of 5 if positive biasing of the grid prevented ions from reaching the fluorocarbon film surface in a CHF3 plasma at 2 mTorr. The energy distribution of the ions which may be obtained from these data is in good agreement with measurements of the plasma potential. The ion fluxes for CF4 are =4-5 times greater than the fluorine and carbon atom fluxes required to explain the deposition rates (assuming a sticking coefficient of 1). Film-growth due to direct ion incorporation rather than ion enhancement can explain the experimental results. For CHF3 plasmas the deposition rates are = 100 nm/min greater than for CF4 for all conditions. This suggests that neutrals contribute strongly to fluorocarbon film growth for CHF3 since the ion currents are nearly the same as for CF4. The ion enhancement effect of film growth rate decreases at higher pressure and lower microwave powers and mirrors the behavior of the ion current. This finding has important implications for etch selectivity, etching profiles, and the slow-down of the Si02 etch rates in high-aspect ratio contact holes. Biasing the substrate reduces the net fluorocarbon deposition rate for low rf bias values. At higher rf bias values, etching of the initially deposited film takes place. These threshold voltages for etching are higher for CHF3 than CF4, e.g., 55 versus 35 V for 1 mTorr operation. Oxide etching can only take place for rf bias values equal or greater than these threshold voltages. © 1994, American Vacuum Society. All rights reserved.