The arrayed waveguide grating is one of the key devices in integrated wavelength division multiplexing optical networks. In order to realize high performance arrayed waveguide gratings, it is essential to optimize the fabrication process of silica-on-silicon planar optical waveguides. A new process for filling the gaps of plasma enhanced chemical rapor deposition(PECVD) silica waveguides using boron-germanium co-doped upper-cladding and high-temperature annealing is proposed. And the measured transmission loss of an arrayed waveguide grating is reduced by about 2 dB. By reason of avoiding the use of toxic phosphine, the proposed method has potential advantages compared with the commonly used borophosphosilicate glass (BPSG) processing. Every step of fabrication, such as photolithography, PECVD, inductively coupled plasma (ICP) and high-temperature thermal annealing, is modified and improved. The excess loss of fabricated arrayed waveguide grating is about 1.5 dB.