[en] A two-step positron facility using a multi-GeV electron beam is under study for electron-positron colliders as ILC or the SuperB factory. In the first step, the electron beam impinges on an oriented tungsten crystal. Channelling radiation provides a large number of γ-rays and consequently creates a large number of pairs in an amorphous target placed after the crystal. Such a method has been successfully tested at CERN and KEK and is now operating at KEK. As a second step and as an upgrade of the system, the electron beam, instead of being sent to the crystal, collides with a circularly polarized laser beam, creating circularly polarized Compton backscattered γ-rays, which impinge on a thin amorphous target where they generate longitudinally polarized positrons. Simulation results are provided for the two steps and the choice of the parameters is discussed for ILC and for the SuperB factory. Particularly, the choice of an ERL instead of a classical linac is recommended for the ILC