We use the three-dimensional finite-difference time-domain (3D-FDTD) method to model silica tapermicrofiber structures integrated on substrates. The dependence of the transmission on the length of the microfiber is investigated for two different structures. Optimization of the geometric parameters is provided and two substrate materials, namely MgF2 and fluorosilicate glass, are considered. We also investigate the case where the structure is covered with a dielectric material.