X-ray photoemission spectroscopy characterization of silicon surfaces after CF4/H2 magnetron ion etching: Comparisons to reactive ion etching

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Abstract
Silicon surfaces subjected to CF4/H2 magnetron ion etching (MIE) have been characterized by x-ray photoemission spectroscopy. Low-power-density (0.274 W/cm2) 25-mTorr MIE plasmas cause fluorocarbon films on Si which are characterized by C–CFx, CF, CF2, and CF3 groups. Graphitic carbon and silicon carbon type bonding can also be observed at the film/silicon interface. Thicker films are formed in hydrogen-rich CF4/H2 etching gas mixtures. Carbonaceous films with significant chemical differences are produced under high-power/low-pressure MIE operating conditions (1 W/cm2 at 4 mTorr). These films are characterized primarily by C–C/C–H type carbon and a very low intensity of highly fluorinated carbon groups. The carbon/fluorine ratio is 0.8 for the films formed in a pure CF4 magnetron discharge at a power density of 0.27 W/cm2, similar to that of films formed in a conventional reactive ion etch plasma, whereas it is 2.2 for the films formed at a power density of 1 W/cm2. The carbonaceous film formed during high-power MIE does not have a dominant role in determining the silicon etching behavior in contrast to fluorocarbon films formed under low-power MIE conditions which suppress the Si etch rate. This explains the lack of SiO2/Si etch selectivity observed under high-power MIE conditions using CF4/H2 in contrast to low-power magnetron ion etching where SiO2/Si etch selectivity is achieved. The formation of carbon-rich film in high-power MIE can qualitatively be explained by a large flux of highly dissociated CF4 derived plasma species to the silicon surface under high-power magnetron ion etching conditions and a Si substrate temperature difference of more than 100 °C between high-power MIE (>200 °C) and low-power MIE (near 100 °C or less) processing.

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