Method of selectively etching first region made of silicon nitride against second region made of silicon oxide

A method of etching a first region made of silicon nitride selectively against a second region made of silicon oxide, comprising:

• preparing a processing target object having the first region and the second region within a chamber provided in a chamber main body of a plasma processing apparatus;
• generating plasma of a first gas including a gas containing hydrogen within the chamber to form a modified region by modifying a part of the first region with active species of the hydrogen;
• and
• generating plasma of a second gas including a gas containing fluorine within the chamber to remove the modified region with active species of the fluorine,
• wherein a plurality of sequences each of which includes the generating of the plasma of the first gas and the generating of the plasma of the second gas are performed in sequence,
• the processing target object further has a third region made of silicon,
• the first region is provided to cover the second region and the third region before the plurality of sequences are performed,
• the plurality of sequences include one or more first sequences performed until a time immediately before the third region is exposed or until the third region is exposed;
• and one or more second sequences performed to oxidize a surface of the third region after the one or more first sequences, and
• the first gas further includes a gas containing oxygen in at least one second sequence.