A semiconductor structure is provided. The semiconductor structure includes metallization structure, a plurality of conductive pads, and a dielectric layer. The plurality of conductive pads is over the metallization structure. The dielectric layer is on the metallization structure and covers the conductive pad. The dielectric layer includes a first dielectric film, a second dielectric film, and a third dielectric film. The first dielectric film is on the conductive pad. The second dielectric film is on the first dielectric film. The third dielectric film is on the second dielectric film. The a refractive index of the first dielectric film is smaller than a refractive index of the second dielectric film, and the refractive index of the second dielectric film is smaller than a refractive index of the third dielectric film.