Plasma etch is preffered because of its anisotropy. To understand why, the limitations of isotropic etching are shown in the picture below:

• for small structures, the resist can be lifted due for isotropical etch
• for small openings the etchant cannot etch anymore
• the structures have modified shape, no vertical walls as desired

• Dry etching advantages over wet etching
  • Better control - easy to start and stop, just by closing the plasma (no bias applied)
  • Reproducible - not so dependent on temperature as is wet etching
  • High anisotropy - smaller structures can be made, and with vertical walls
  • Less chemical waste