[CCoE Notice] PhD Defense: Optical Spectroscopy of CH3F/O2, CH3F/CO2 and c-C4F8 Inductively Coupled Plasmas

[Grayson, Audrey A]

PhD DEFENSE STUDENT: Qiaowei Lou
DATE: Monday,July 31, 2017
TIME: 1:00 PM
PLACE:  Mechanical Engineering Large Conference Room
DISSERTATION CHAIRs:
Dr. Vincent Donnelly and Dr. Demetre Economou
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TITLE:
Optical Spectroscopy of CH3F/O2, CH3F/CO2 and c-C4F8 Inductively Coupled Plasmas

Despite the fact that CH3F/O2 and CH3F/CO2 gas mixtures are widely used in the microelectronics industry for highly anisotropic and selective etching of Si3N4 over Si or SiO2, only a limited number of fundamental plasma studies of these gases have been reported. Recently, an inductively coupled plasma (ICP) beam system was used in this laboratory to study fundamental aspects of this etching process. The topic of this thesis was to characterize that plasma beam, operated with CH3F/CO2 feed gases, as a function of power (5-400 W) and gas composition at 10 mTorr, using optical emission spectroscopy and rare gas actinometry. Number densities of H, F, and O increased rapidly between 74-80% CO2, ascribed to the transition from polymer-covered to polymer-free reactor walls, similar to that in CH3F/O2 ICPs at 48% O2. The plasma composition of the two gas mixtures were almost identical below 40% O2 or CO2, except for higher OH/Xe over the full range of CH3F/CO2 composition. CO number densities were obtained by self-actinometry. CO2 was little dissociated below 40 W while the dissociation increased with power to ~83% at 400 W. A model from previous work was used to predict F/Ar number density ratios and estimate the probability (γ) of hydrogen abstraction from the wall by F atoms.
The species concentrations measurements performed in this study are used to explain the faster deposition rates and other properties, observed in other studies, of fluorocarbon (FC) films on p-Si exposed to CH3F/CO2 compared to CH3F/O2 plasma beams emanating from an ICP source, used in separate etching studies.
A C4F8 inductively coupled plasma in pulse mode was investigated as a function of peak power (25-400 W), pulse repetition frequency (5-100 kHz), and duty cycle (10-90%) to support a study of atomic layer etching of SiO2. Relative emission intensity ratios of C/Xe, CF/Xe, CF2/Xe, C2/Xe and F/Ar were obtained and analyzed. All ratios were underestimated at high powers and duty cycles, except for F/Ar, due to the energy mismatch. The CF2 emission from electron impact dissociative excitation was dominant below 50 W, but became much less important at high powers. A decrease of the emission intensity ratios of all species was observed with increasing frequency. CF2 number density measured by UV absorption spectroscopy was found to be nearly constant at ~2.5x1014 cm-3 between 150~450 W. The highest CF2/F density ratio, required in atomic layer etching for self-limiting deposition of a thin FC film growth, was achieved at a peak power of 50 W, for pulse repetition frequency of 10kHz, duty cycle of 50%, and pressure of 6.5 mTorr. This near self-limiting deposition of a ~2 monolayer thick FC film also required producing a plasma beam with a CF2 flux that was much higher than that of the sum of CF and C fluxes. Thus, the reported large sticking coefficient of CF2 on bare SiO2, combined with the much lower sticking coefficient on Teflon-like films produced from the CF2 precursor, will result in a near-self-limiting FC film deposition.
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