Sensing structure, sensing chip including the same, and method for forming the same

A sensing structure, comprising:

• a substrate having a first recess;
• and
• a metal layer disposed on the substrate, comprising:
• a first portion disposed on a top surface of the substrate;
• and
• a second portion disposed in the first recess and comprising:
• a base portion;
• and
• a protruding portion disposed on the base portion and extending away from the substrate,
• wherein a gap is between the first portion and the protruding portion and surrounds the protruding portion, and the gap is in a range of 20 nm to 100 nm,
• wherein the substrate has a second recess adjacent to the first recess, and a pitch between the first recess and the second recess is in a range of 250 nm to 450 nm,
• wherein the pitch is greater than a width of the base portion, and the width of the base portion is in a range of 120 nm to 250 nm, and
• wherein the width of the base portion is greater than a width of the protruding portion, and the width of the protruding portion is in a range of 40 nm to 250 nm.