ANISOTROPIC NITRIDE ETCHING HAVING HIGH SENSITIVITY WITH RESPECT TO OXIDE AND PHOTORESIST LAYER

PROBLEM TO BE SOLVED: To pattern a silicon nitride layer so that it has a high aspect ratio by etching an exposed part of the silicon nitride layer with a high density plasma generated by exciting an etchant gas which includes a polymerizing agent, a source of hydrogen, an oxidant, and a noble gas diluent, to form a trench. SOLUTION: An etchant gas includes a polymerizing agent, a source of hydrogen, an oxidant, and a noble gas diluent. The polymerizing agent is a precursor for causing formation of passivation layer and is preferably selected from among CF4, C2F6, and C3F8. The source of hydrogen is preferably selected from among CHF3, CH2F2, CH3F, and H2, and the oxidant is selected among CO, CO2, and O2. This etchant gas is excited to generate a high density plasma. A part of a silicon nitride layer 131 is exposed by an etch window 133 to etch the exposed part of the silicon nitride layer 131 with the plasma, to form a trench which extends to a silicon oxide layer.