Picosecond ultrasonic study of Mo/Si multilayer structures using an alternating-pump technique

We have used picosecond ultrasonics techniques to study the localized acous tic-phonon surface modes in Mo/Si multilayer reflectors for extreme ultravi olet lithography. Localized surface modes in the first (zone-boundary) and second (zone-center) gaps were simultaneously detected. Oscillation frequen cy as high as 0.873 THz was observed. An alternating-pump technique has bee n successfully demonstrated to enhance the signal-to-noise ratio by 10 dB. This technique can be used to improve the sensitivity for probing the surfa ce modes in other multilayer thin-film structures. (C) 1999 American Instit ute of Physics. [S0003-6951(99)03302-1].