A substrate is provided. The substrate includes a plurality of devices disposed in the substrate, a plurality of contact pads disposed on a surface of the substrate and electrically connected to the devices, and a surface dielectric layer positioned on the surface of the substrate. Thereafter, a surface treatment process including at least a plasma etching process is performed. Subsequently, at least a plasma enhanced chemical vapor deposition (PECVD) process is performed to form a dielectric layer on a surface dielectric layer. The PECVD process is performed in a high frequency/low frequency alternating manner. Following that, a masking pattern on the dielectric layer is formed, and an anisotropic etching process is carried out to form a plurality of openings corresponding to the contact pads in the dielectric layer. The openings expose the contact pads, and each opening has an outwardly-inclined sidewall.