Superconducting single photon detectors (SSPD) made of superconducting NbN ultrathin film were successfully synthesized down to 70 nm nanowire width. Their fabrication process is detailed, with emphasis on electron beam lithography. A high-resolution electron beam lithography tool was operated at 100 kV in combination with a negative tone resist NEB22A2, in order to achieve the required resolution and fabrication yield. The sensitivity of ultrathin NbN films to various resist stripping treatments is evidenced by electrical transport measurements, and circumvented by deposition of adequate protecting layers. SSPDs made of a 4.4 nm thick NbN film were electrically characterized, and show a critical current density above 3x10(6) A cm(-2) and a critical temperature above 11 K. (c) 2007 American Vacuum Society.