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Etching mechanism of the single-step through-silicon-via dry etch using SF6/C4F8 chemistry

A single-step etching method using the SF6/C4F8 chemistry is developed in this study as an alternative through-silicon-via (TSV) etching approach of the traditional Bosch process to realize ultrasmooth and vertical TSV profiles. Experimental results show that there is a profile discontinuity, or a “transition,” on the TSV profile produced by the single-step etching method at high bias voltages and high SF6 flow rates. Comparison between the intensity of the species generated in a pure SF6 or a pure C4F8 plasma and in a SF6/C4F8 plasma is investigated for better understanding interactions between SF6 and C4F8. The densities of all positive ions are reduced in the SF6/C4F8 plasma compared to a pure SF6 plasma and a pure C4F8 plasma at the same partial pressure, indicating a change of plasma chemistry when SF6 and C4F8 fluxes are mixed. The formation mechanism of the transition is proposed as a chemistry discontinuity caused by large-angle ion sputtering at the top part of the sidewalls and the polymer accumulation at the bottom part of the sidewalls. The formation of the transition has found to have an effect of improving the sidewall smoothness below the position where it is formed. Parameter study has shown that a decreased bias voltage and a reduced SF6/C4F8 ratio can help to improve the sidewall smoothness and eliminate the transition on the TSV profiles.