High voltage electron beam lithography experiments on thin films of tungsten trioxide are reported. For linear doses around 10 mC/cm at 200 kV, a negative resist behavior is observed after development in a NaOH solution. Features with dimensions as low as 15 nm with aspect ratios up to 10, exhibiting weak size fluctuations and vertical sidewalls are routinely achieved. Proximity effects are found to be negligible allowing the fabrication of structures with less than 10 nm separation. The exposure mechanism is investigated by transmission electron microscopy and diffraction. A crystallization process induced by the electron bombardment is clearly evidenced. A model based on the amorphous to crystalline phase transition induced by electron irradiation is proposed to account for the experimental observations.