Plasma atomic layer etching of SiO2 and Si3N4 with heptafluoropropyl methyl ether (C3F7OCH3)
In this work, atomic layer etching (ALE) with heptafluoropropyl methyl ether (C3F7OCH3) plasma was developed for SiO2 and Si3N4 and compared with the results of C4F8 or CHF3 plasmas. C3F7OCH3 has a shorter life time and lower global warming potential (GWP) than CHF3 and C4F8. SiO2 and Si3N4 surfaces were fluorinated with fluorocarbons generated from C4F8 or CHF3 or C3F7OCH3 plasmas, and the fluorinated surface was then removed by ions or radicals generated from Ar or O2 plasma in the following step. Atomic scale etch rates were achieved with cyclic etch rates of 5.8 Å/cycle for C4F8/Ar, 4.1 Å/cycle for CHF3/Ar, and 2.1 Å/cycle for C3F7OCH3/Ar. In case of etching with oxygen, atomic scale etch rates were achieved with cyclic etch rates of 2.9 Å/cycle for C4F8/O2, 1.7 Å/cycle for CHF3/O2, and 1.1 Å/cycle for C3F7OCH3/O2. The etch rate was correlated with the F1s/C1s ratio of the fluorocarbon layers; C3F7OCH3 plasmas generated fluorocarbon layers having the lowest F1s/C1s ratio, and C4F8 plasmas produced the highest F1s/C1s ratio. Constant etch rates were observed in the bias voltage range of 55–60 V, which is identified as the ALE window. In the etching step, Ar and O2 plasmas were applied to remove the fluorocarbon layers. A saturated etch rate with etching time, i.e., a self-limited etching rate, was obtained for all the fluorocarbon gases with both Ar and O2 plasmas. The high etch selectivity of 17.5 was achieved for SiO2/Si and 26.6 for Si3N4/Si with C3F7OCH3/Ar. These high selectivities are attributed to Si–C bonds that act as inhibitors during Si etching.
