Characterization of SiO2 Etching Profiles in Pulse-Modulated Capacitively Coupled Plasmas

Although pulse-modulated plasma has overcome various problems encountered during the development of the high aspect ratio contact hole etching process, there is still a lack of understanding in terms of precisely how the pulse-modulated plasma solves the issues. In this research, to gain insight into previously observed phenomena, SiO2 etching characteristics were investigated under various pulsed plasma conditions and analyzed through plasma diagnostics. Specifically, the disappearance of micro-trenching from the use of pulse-modulated plasma is analyzed via self-bias, and the phenomenon that as power off-time increases, the sidewall angle increases is interpreted via radical species density and self-bias. Further, the change from etching to deposition with decreased peak power during processing is understood via self-bias and electron density. It is expected that this research will provide an informative window for the optimization of SiO2 etching and for basic processing databases including plasma diagnosis for advanced plasma processing simulators.

FEATURES OF SiO2 REACTIVE-ION ETCHING KINETICS IN CF4 + Ar + O2 AND C4F8 + Ar + O2 GAS MIXTURES

The effect of Ar/O2 mixing ratio on gas-phase characteristics and SiO2 etching kinetics in CF4 + Ar + O2 and C4F8 + Ar + O2 plasmas was studied under conditions of 13.56 MHz inductive RF discharge. The constant processing parameters were fraction of fluorocarbon component in a feed gas (50%) total gas pressure (6 mTorr), input power (700 W) and bias power (200 W). It was found that the full substitution of Ar for O2 in both gas systems results in non-monotonic (with a maximum at ~ 25% Ar + 25% O2) SiO2 etching rates as well as in monotonically increasing photoresist etching rate with higher absolute values for CF4-containing mixture. The steady-state densities of active species were determined using a combination of plasma diagnostics by Langmuir probes and 0-dimensional (global) plasma modeling. Corresponding results indicated that both gas systems are characterized by quite close parameters of electron and ion components while exhibit sufficient differences in the kinetics of neutral species, especially in the presence of O2. The latter produces opposite changes in F atom density as well as in effective probability of ion-assisted chemical reaction vs. Ar/O2 mixing ratio. Relationships between type of fluorocarbon component and heterogeneous process kinetics were analyzed through the set of gas-phase-related parameters (fluxes, flux-to-flux ratios) characterizing chemical etching pathways for SiO2 and formation/destruction balance for the fluorocarbon polymer film. It was suggested that the transition toward O2-rich plasma in the low-polymerizing CF4 + Ar + O2 plasma suppresses the effective probability for SiO2 + F reaction through decreasing efficiency for oxide bond breaking and desorption of etching products due to decreasing ion energy flux. Oppositely, an increase in O2 content in the high-polymerizing C4F8 + Ar + O2 mixture lifts up the effective reaction probability by decreasing fluorocarbon film thickness and providing better access of F atoms to the etched surface.

PLASMA PARAMETERS AND SiO2 ETCHING KINETICS IN C4F8 + Ar + O2 GAS MIXTURE

The effect of Ar/O2 mixing ratio on plasma parameters, steady-state densities of active species and SiO2 etching kinetics in the three-component C4F8+Ar+O2 gas mixture was studied under typical conditions of reactive ion etching process (inductive 13.56 MHz RF discharge, total gas pressure of 6 mTorr, input power of 700 W and bias power of 200 W). The investigation combined etching rate measurements, plasma diagnostics by Langmuir probes and 0-dimensional (global) plasma modeling in order to determine steady-state densities and fluxes of plasma active species. It was found that the full substitution of Ar for O2 at constant fraction of fluorocarbon gas (in fact, the transition from 50% C4F8 + 50% Ar to 50% C4F8 + 50% O2 gas system): 1) results in weakly non-monotonic (with a maximum) SiO2 etching rate with close values for both O2-free and Ar-free plasmas; 2) causes the monotonic decrease in both F atom flux and ion energy flux; and 3) suppresses the formation of the fluorocarbon polymer film on the etched surface through its oxidative destruction pathway. The model-based analysis of SiO2 etching kinetics allowed one to conclude that an increase in effective probability for SiO2 + F reaction contradicts with the behavior ion energy flux as well as demonstrate the agreement with the change in gas-phase parameters characterizing the fluorocarbon film thickness. Therefore, an increase in O2 content in a feed gas influences the effective reaction probability by decreasing fluorocarbon film thickness and providing better access of F atoms to the etched surface.