In-Situ Characterization of High Aspect Ratio Plasma Processing Through Manipulation of C₄F₈ Plasma Gas Phase Interactions

Low-pressure fluorocarbon plasmas have proven to be essential in various modern semiconductor fabrication processes, including thin film deposition or dielectric etching depending on the gas mixture, power, and sample distance from the plasma source. However, the continuous pursuit of shrinking the integrated circuit node has presented challenges emphasizing precise control of the plasma species involved in high aspect ratio (HAR) plasma processes. With increasing aspect ratios (AR), the control of electrically neutral plasma species (i.e. radicals) plays a crucial role in the channel critical dimension (CD) of a HAR etched feature. This is attributed with changes in radical composition as a function of channel depth, influenced by gas-phase and gas-surface interactions of radicals throughout the etched channel. As such, in-situ diagnostic techniques must be performed to determine the radical species distribution and composition of the fluorocarbon plasma prior to entering a given HAR etched feature.<br/>For better understanding of such mechanisms and better control over neutral species delivery and distribution inside etch channels, we characterized the transport of radical species in the process chamber as a function of the distance from the plasma source. The experiments were performed in an inductively coupled C₄F₈/O₂/Ar discharge at 20 mTorr to minimize collisions between molecules. The composition of the radicals diffusing toward the sample holder was controlled by varying the sample distance from the plasma source and was monitored in-situ by spatially-resolved optical emission spectroscopy (SROES) and spatially-resolved mass spectrometry (SRMS). The plasma phase chemistry composition and the nature of the radicals transported to the sample holder were controlled by adjusting the O₂ flow rate. Using the sample plasma, experimental films were grown on SiO₂ stacked structures mounted with capillary plates with varied AR. The SiO₂ stacked structure was made to mimic the equivalent of the greatest capillary AR at a millimeter scale to compare surface interaction discrepancies as a function of channel depth. The composition and deposition rates of the films were determined as a function of AR via X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), respectively.<br/>In-situ gas analysis indicated that the C₄F₈ plasma dissociation and recombination rates increased with O₂ flow rate and distance from the plasma source. An O₂ addition threshold that yielded the greatest CF₃concentration and minimal CF and CF₂ production was identified. Surface analysis indicated similarities in film composition between the millimeter and micrometer scale HAR structures. The observed reduction in carbon content film deposition (CF₃ rich surfaces) with the increase of the channel length could be addressed to higher recombination and consumption rates of CF and CF₂ species on the via wall. Additional characterization of the radical flux, its spatial distribution, and its impact on the film properties will be discussed.