In an exemplary embodiment, each of the first extension electrode and the second extension electrode is a circular extension electrode, and an entire area of the circular extension electrode is used to form the plurality of concave spots.

In an exemplary embodiment, surfaces of the first contact electrode and the second contact electrode are flush with each other to constitute a forming surface; each of the first contact electrode and the second contact electrode is a circular contact electrode; and a projected area of the circular extension electrode on the forming surface is larger than a projected area of the circular contact electrode on the forming surface.

In an exemplary embodiment, a projection of the circular extension electrode on the forming surface and a projection of the circular contact electrode on the forming surface constitute a structure of concentric circles.

In an exemplary embodiment, a circular area in each of the first extension electrode and the second extension electrode is used to form the plurality of concave spots.

In an exemplary embodiment, the first contact electrode includes a primary contact electrode, and at least one auxiliary contact electrode is isolated from the primary contact electrode; the first extension electrode includes a primary extension electrode covering the primary contact electrode, an auxiliary extension electrode covering the auxiliary contact electrode, and a linear extension electrode connecting the primary extension electrode with the auxiliary extension electrode; and the plurality of concave spots are formed on a circular area of the primary extension electrode above the primary contact electrode.