Improvement of electron emission of silicon field emitter arrays by pulsed laser cleaning

A diode-pumped a-switched Nd:YLF laser was used for the cleaning of microfabricated silicon field emitter arrays. The laser wavelength was systematically varied from infrared to visible and ultraviolet, while the emission current was measured simultaneously. Infrared (lambda = 1047 nm) or visible (lambda = 523 nm) laser irradiation did not alter the emission behavior. Ultraviolet laser irradiation at lambda = 349 nm, on the other hand, resulted in a significant increase in the emission current due to cleaning of the emitter tip surface. Numerical simulation of the laser induced temperature rise at the silicon surface and the wavelength dependence of the cleaning efficiency indicate that photochemical decomposition, requiring energetic photons, is the main mechanism for the removal of the contaminants. However, if higher energetic photons at lambda = 262 nm (fourth harmonic) are used, a temporary decrease in emission current during laser irradiation was observed. A simultaneous decrease of the vacuum level in the test chamber indicates that this unexpected behavior is attributable to the removal of ionized species from the silicon tip. Removal of the contaminants without ionization, therefore, appears to be an essential condition for efficient cleaning of the emitter tips and correspondingly for an increase in the emission current. (C) 2000 American Vacuum Society. [S0734-211X(00)03502-2].