UHV Ultrafast Scanning Electron Microscopy

Ultrafast Scanning Electron Microscopy (USEM) merges photonics and electron microscopy, combining the temporal resolution of femtosecond laser spectroscopy with the nanoscale spatial resolution and surface selectivity typical of scanning electron microscopy, and provides imaging of dynamical photo induced processes at surfaces and in ultra-thin films. Our USEM apparatus is based on a UHV (10-9÷10-10 Torr) Field-Effect SEM, modified with optical windows at the tip of the electron gun and at the specimen chamber and optically coupled to an ultrafast pulsed fs laser source (300 fs,1030 nm) operated at 10 MHz repetition rate. The UHV controlled environment guarantees elevated sensitivity to the physical-chemistry of sample surfaces. Time-resolved operation is achieved by means of a pump-probe configuration, where the UV third harmonic (TH) beam of the laser source works as the optical pump, while the fourth harmonic (FH) beam promotes the emission of an ultrafast pulsed electron probe beam from the SEM tip. Pump and probe pulses are relatively delayed by an optical delay stage set on the optical path of the pump beam and the pump-probe relative delay is tuned over 4 ns with sub-ps resolution. SE detection is provided by an Everhart-Thorley detector, either directly operated in current mode for time resolved imaging or lock-in demodulated for time spectroscopy on selected areas. A signal rise time of about 10 ps has been demonstrated. We will discuss the state-of-the art of the technique, its key parameters and domains of application. We will present the temporal evolution of photoinduced SE contrast under strong UV optical pumping regime, providing information on optically excited dynamics of charge carriers and defects at surfaces of semiconductors and oxide overlayers.