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 ellipsometrically 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 SiO2 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.

We report a study of the application of CF4 and CHF3 electron cyclotron resonance (ECR) discharges to selective etching of SiO2 over Si. Due to significant fluorocarbon film deposition for plasma operation without rf sample bias in the pressure range below 10 mTorr, rf biasing is required for etching of SiO2 and Si. The rf threshold voltage for etching is 55 V for CHF3 and 35 V for CF4 at a pressure of 1 mTorr. At 100 V rf bias, silicon dioxide etch rates were greater than ≂600 nm/min in CF4 and 450 nm/min for 1000 W plasmas at 1 mTorr pressure. A plot of the oxide etch rate vs rf bias exhibits a fluorocarbon film suppression regime at low rf voltages and an oxide sputtering regime at higher rf voltages. In the fluorocarbon suppression regime, the etch rate is primarily determined by fluorocarbon deposition which results in a thin fluorocarbon film being present on the SiO2 surface during steady-state etching. In the oxide sputtering regime, the oxide etch rate increases linearly with the ion current to the wafer and the square root of the ion energy. The etch yields decrease with increasing microwave power and decreasing pressure and are in the range 0.5–2 atoms per incoming ion. The silicon etch rate is much lower in CHF3 than in CF4, which translates into better SiO2-to-Si etch selectivity in CHF3 (≂15) than in CF4 (≂5). The lower Si etch rate in CHF3 is due to a greater thickness of the fluorocarbon film present on the silicon surface during steady-state etching. The fluorocarbon film thickness is ≂5.5 nm in CHF3 as compared to ≂2.5 nm in a CF4 discharge (at a rf bias of 100 V). The oxide surface is free of fluorocarbon film for the same conditions. The etch depth of ≂2.5 μm deep contact holes etched using 1 mTorr CHF3 plasmas into photoresist patterned SiO2 was measured by scanning electron microscopy as a function of the feature width. The etch depth decreased by ≂10% as the feature size was reduced from 1.3 to 0.6 μm.

A recent, important development in low-pressure plasma processing is the radio frequency inductively (RFI) coupled high density discharge. Its ability to create high densities of excited and charged species at low pressures (<10−3 Torr) makes it an attractive etching tool. In this work we have examined selective etching of SiO2 over Si using a home-built RFI source. CHF3, C2F4, C3F6 and their mixtures with hydrogen were examined. Without biasing of the substrate strong fluorocarbon deposition occurs over the investigated pressure range from 5 to 20 mTorr. As the pressure increases the ion current density decreases, whereas the fluorocarbon deposition rate increases. Both parameters increase roughly linearly with inductive rf power from 500 up to 1250 W. Etching was achieved by rf biasing. When the pressure is reduced from 20 to 6 mTorr, the oxide and silicon etch rates decrease less than 20% for all gases. The highest oxide etch rate of 830 nm/min at 350 W rf bias power is achieved for C3F6. Adding H2 decreases the etch rates for oxide and silicon for all gases. The drop of the silicon etch rate is considerably higher than for the oxide etch rate resulting in a better selectivity. The best selectivity of 45 is achieved for C2F4 when 30% H2 is added into the discharge. The results obtained with the RFI source are compared to results with a microwave electron cyclotron resonance discharge.

Etching of high aspect ratio patterns induces a phenomenon known as reactive ion etching (RIE) lag, i.e., a large feature etches faster than a smaller one. This effect is studied for oxide etched in a high density plasma excited by electron cyclotron resonance using different fluorocarbon gases. The magnitude of the RIE lag is correlated with the deposition rate of fluorocarbon film on an unbiased sample, showing that chemical effects are important to understand the mechanisms of RIE lag in high density plasmas.

Recent selective oxide etching results obtained with an electron cyclotron resonance high-density plasma reactor are presented. Reactive ion etching lag results of patterned SiO2 samples etched with various fluorocarbon gases are discussed. A reactive ion etching lag mechanism which is based on the dependence of the oxide etch rate on rf power is proposed.