Contributions of gas-phase plasma chemistry to surface modifications and gas-surface interactions: investigations of fluorocarbon rf plasmas
Date
2012
Authors
Cuddy, Michael F., author
Fisher, Ellen R., advisor
Levinger, Nancy E., committee member
Rickey, Dawn, committee member
Krummel, Amber, committee member
Yalin, Azer P., committee member
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Abstract
The fundamental aspects of inductively coupled fluorocarbon (FC) plasma chemistry were examined, with special emphasis on the contributions of gas-phase species to surface modifications. Characterization of the gas-phase constituents of single-source CF4-, C2F6-, C3F8-, and C3F6-based plasmas was performed using spectroscopic and mass spectrometric techniques. The effects of varying plasma parameters, including applied rf power (P) and system pressure (p) were examined. Optical emission spectroscopy (OES) and laser-induced fluorescence (LIF) spectroscopy were employed to monitor the behavior of excited and ground CFx (x = 1,2) radicals, respectively. Mass spectrometric techniques, including ion energy analyses, elucidated behaviors of nascent ions in the FC plasmas. These gas-phase data were correlated with the net effect of substrate processing for Si and ZrO2 surfaces. Surface-specific analyses were performed for post-processed substrates via x-ray photoelectron spectroscopy (XPS) and contact angle goniometry. Generally, precursors with lower F/C ratios tended to deposit robust FC films of high surface energy. Precursors of higher F/C ratio, such as CF4, were associated with etching or removal of material from surfaces. Nonetheless, a net balance between deposition of FC moieties and etching of material exists for each plasma system. The imaging of radicals interacting with surfaces (IRIS) technique provided insight into the phenomena occurring at the interface of the plasma gas-phase and substrate of interest. IRIS results demonstrate that CFx radicals scatter copiously, with surface scatter coefficients, S, generally greater than unity under most experimental conditions. Such considerable S values imply surface-mediated production of the CFx radicals at FC-passivated sites. It is inferred that the primary route to surface production of CFx arises from energetic ion bombardment and ablation of surface FC films. Other factors which may influence the observed CFx scatter coefficient include the surface with which the radical interacts, the vibrational temperature (ΘV) of the radical in its gas phase, and radical interactions in the gas phase. The analyses of ΘV in particular were extended to diatomic radicals from other plasma sources, including nitric oxide and fluorosilane systems, to gauge the contributions of vibrational energy to surface reactivity. In general, a monotonic increase in S is observed for CF, NO, and SiF radicals with increasing ΘV. Preliminary results for mixed plasma precursor systems (i.e. FC/H2, FC/O2) indicate that the choice of feed gas additives has a profound effect on surface modification. Hydrogen additions tend to promote FC film deposition through scavenging of fluorine atoms, whereas oxygen consumes polymerizing species, thus favoring etching regimes. Time-resolved optical emission spectroscopy (TR-OES) studies of gas-phase species elucidate the mechanisms by which these processes occur. Ultimately, the work presented herein expands the fundamental chemical and physical understanding of fluorocarbon plasma systems.
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Subject
fluorocarbon film deposition
vibrational temperature
surface reactivity
fluorocarbon plasmas
plasma ion energy distributions
CFx plasma kinetics