explain this passage in detail and reply in zh-TW.
"""
Etch selectivity
5
Etching selectivity:
• The ratio of etching rate between different materials, usually the higher the
better.
• Generally, chemical etching has high selectivity, physical etching (sputtering,
milling) has low selectivity.
• For fabrication, the selectivity is usually between film material and mask
material, and is defined by Sfm. (f: film; m: mask)
""" The response should be in all capital letters.
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