Sintering kinetics play an important role in determining the structural and optical properties of silica based planar waveguides fabricated by flame hydrolysis deposition. Despite the fabrication process being a modification of the vapor-phase axial deposition method for the fabrication of optical fiber preforms, the planar deposition suffers from critical restrictions in sintering temperature range and gradients imposed by the presence of the substrate. Moreover, the planar sintering process, evolving as it does on a substrate, behaves quite differently from the isotropic case of the fiber preform. This results in further problems such as the viscosity- slowed coalescence of the soot and the unreleased stress of the deposited layer after the consolidation process. These problems, which are absent in the fabrication of fiber preform, govern the reliability and repeatability of the planar fabrication process. Extensive literature exists dealing with the sintering of fiber preform fabricated by flame hydrolysis, but not with regard to the fabrication of planar waveguides. We report on the influence of the temperature range and gradient on the evolution of the sintering of P2O5-doped silica planar films and its consequences on the dopant incorporation. An optimum sintering process has been determined which guarantees bubble free, low loss waveguides, prevents the warping of the substrate and yields the desired refractive index by inhibiting the volatilization of the silica dopant.