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Comparison of the effects of downstream H2- and O2-based plasmas on the removal of photoresist, silicon, and silicon nitride

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3 Author(s)
Thedjoisworo, Bayu ; Lam Research, Surface Integrity Group (SIG), 4000 North 1st St., San Jose, California 95134 ; Cheung, David ; Crist, Vince

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For the 45 nm technology node and beyond, there is a need to strip photoresist quickly while suppressing the loss of materials such as polycrystalline silicon (poly-Si) and silicon nitride (Si3N4). To achieve this goal, the authors characterized and compared the effects of downstream pure-H2, H2/N2, and O2/N2 plasmas on the etch behaviors of photoresist, poly-Si, and Si3N4. The addition of N2 to H2 plasma increases the photoresist ash rate to a maximum that is reached at ∼30–40% N2, and the ash rate drops with further addition of N2. At 30% N2 addition, the ash rate increases by a factor of ∼3 when compared to that obtained with pure-H2 plasma. For O2/N2 plasma, the photoresist ash rate also exhibits a maximum, which is attained with 5% N2 addition, and the ash rate drops drastically as more N2 is added. A small addition of N2 increases the H and O radical densities in the H2- and O2-based plasmas, respectively, resulting in the higher ash rates. The ash rate achieved by the O2/N2 chemistry is generally higher than that attained with the H2/N2 chemistry, and the difference becomes more significant at high temperatures. The activation energy for photoresist strip under O2/N2 plasma was measured to be ∼10 kcal/mol, which is higher when compared to the ∼5 kcal/mol measured for both the H2/N2 (30% N2) and the pure-H2 chemistries. At 300 °C, when compared to the O2-based chemistry, the H2-based chemistry was shown to remove Si3N4 with a much lower rate, ∼0.7 Å/min, highlighting the b- nefit of the latter in conserving material loss. The ability of the H2-based chemistry to suppress material loss and its nonoxidizing property could justify the trade off for its lower ash rates when compared to those obtained using the O2-based chemistry. For the H2-based chemistry, a small N2 addition to the H2 plasma was found to not only increase the ash rate but also suppress the Si etch rate by a factor of 8 to 22, depending on the temperature. Collectively, the H2/N2 chemistry shows a great promise for photoresist-strip applications in the advanced nodes, and it should be run at high temperatures (e.g., T ≥ 300 °C) to maximize the ash rate while still maintaining extremely low Si and Si3N4 losses.

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Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures  (Volume:31 ,  Issue: 2 )