Technique to deposit sidewall passivation for high aspect ratio cylinder etch

An apparatus for forming an etched feature in a stack on a semiconductor substrate, the stack comprising a dielectric material, the apparatus comprising:

• one or more reaction chambers, wherein at least one reaction chamber is configured to perform etching and comprises a plasma generator, and wherein at least one reaction chamber is configured to perform deposition, each reaction chamber comprising:
• an inlet for introducing process gases to the reaction chamber, and
• an outlet for removing material from the reaction chamber;
• and
• a controller configured to cause:
• (a) generating an etching plasma comprising an etching reactant, exposing the substrate to the etching plasma, and partially etching the feature in the stack, wherein (a) is performed in the reaction chamber configured to perform etching;
• (b) after (a), depositing a protective film on sidewalls of the feature by
• (i) exposing the substrate to a first reactant and allowing the first reactant to adsorb onto the substrate,
• (ii) exposing the substrate to a second reactant, wherein the first and second reactants react with one another to form the protective film, and
• (iii) repeating (i) and (ii) in a cyclic manner until the protective film reaches a target thickness, wherein the protective film is an organic polymeric film and is deposited along substantially the entire depth of the feature, and wherein (b) is performed in the reaction chamber configured to perform deposition;
• (c) repeating (a)-(b) until the feature is etched to a final depth, wherein the protective film deposited in (b) substantially prevents lateral etch of the feature during (a), and wherein the feature has an aspect ratio of about 5 or greater at its final depth.