We studied a method for increasing the etching rate of SiN films in CF4/O-2 downflow plasma. It was confirmed that the etching rate of SiN films deposited by plasma-enhanced chemical vapor deposition (PE-CVD) was 50-200 times higher than that of the films deposited by low-pressure chemical vapor deposition (LP-CVD). The activation energy for PE-CVD SiN (PE-SiN) was 0.62 kcal/mol and that for LP-CVD SiN (LP-SIN) was large, 5.3 kcal/mol, indicating that different etching reactions occurred. Assuming that the difference is caused by the many H atoms contained in PE-SiN, we compared the etching rates by adding gaseous H-2, water, and methanol, which include H atoms, to CF4/O-2 The results show that the etching rate increased by a maximum factor of 10 for LP-SiN and approximately a factor of 1.5 times for PE-SiN, From these results, we propose the following etching mechanism for the enhancement in the etching rare. Hydrogen added to CF4/O-2 reacts with N atoms on the SiN films to form NH3, which is then removed as a volatile component. With the removal of these N atoms, the SiN surface becomes Si rich; therefore, the etching reaction of Si atoms and the F radicals is accelerated, thereby increasing the etching rate.