Laser pulse‐induced transitions on surfaces of bulk material, traced by thermal and secondary electrons
Authors: Bostanjoglo O. , Heinricht F. 
Published: August 2011

Thermal and electron beam‐released electrons were exploited to probe the dynamics of surface modifications, induced by a Q‐switched frequency‐doubled Nd:YAG laser within areas of 100 μm Φ on bulk silicon and metals. Changes of surface geometry and phase transitions show up as pronounced peaks and steps in the emitted electron currents. They occur within Φ 200 ns after the laser pulse and consume times from less than 5 ns up to 200 ns.

Applications of pulsed radiation power for surface machining, recrystallization and vitrification or for production of microstructures on bulk substrates for integrated devices are continually extending (Bäuerle 1984). Laser pulses are most frequently used, in spite of their inhomogeneous absorption, as contrasted to electron and ion beams, because of easy handling of power and no need for vacuum in many cases. The diagnostic methods usually applied to probe the dynamics of pulse‐induced transitions are still those introduced at the beginning of the “laser annealing” technique, exploiting light reflection and transmission, and to a minor extent x‐ray and electron diffraction, mass spectrometry and electrical conductivity (Khaibullin 1984, Larson 1984). Naturally, each diagnostic tool has a restricted range of useful application, so further methods combining high temporal and spatial resolution and equally applicable to semiconductors and metals are highly desirable.

Thermal and secondary electrons are expected to be susceptible to changes of temperature and geometry of a surface, occuring during laser pulse machining. In fact, photon‐assisted thermal electron emission was used to probe thermal relaxations in laser‐pulsed semiconductors (Leung and van Driel 1984), however, the thermal electrons were not used to trace geometric modifications. Furthermore, secondary electrons have not yet been tested as a probe for very fast single effects. This report describes first results, demonstrating the usability of emitted electrons as a probe for nanosecond transitions on surfaces of bulk material.

An electron optical equipment was built, consisting of electron gun, condensor lens and specimen chamber, allowing synchronized laser and electron beam pulsing of the specimen. Its surface was probed by the photo‐thermally emitted electrons and ions or by the secondary and backscattered electrons, which were generated by the focussed primary electron beam. The latter was pulsed in order to suppress electron radiation damage. For laser treatment a pulse of a Q‐switched frequency‐doubled Nd:YAG laser (FWHM 20 ns) was focussed with a lens and a dielectric mirror onto the specimen, which could be viewed with a microscope for aligning the laser and the primary electron beam. Both beams had a diameter of 100 μm (FWHM) on the specimen. The emitted charges were collected by a shielded scintillator/multiplier detector of the Everhart‐Thornley type, having a rise time of = 3 ns and being protected against excessive green laser light by an edge filter. Despite the simp1e set‐up, rapid changes of the surface by vaporization, melting, solidification could readily be observed within areas down to 30 μm Φ on the nanosecond time scale. Flow and disrupture of liquid layers occur after the laser pulse, delayed by several 10 ns (Figs. 1 and 2). The dispersion of metal liquids by temperature‐induced gradients of the surface tension may consume times from below 5 ns (Fig. 2a) up to 200 ns (Fig. la). It is signalized by a large increase of electron emission, probably due to Schottky effect at charged transient tips within the disintegrating liquid. First order phase transitions involving latent heats are readily indicated by thermal electrons. Solidification of a melt, for instance, shows up in the emission current as a prolonged plateau with an abrupt drop within 20 … 40 ns (Fig. 3). Summarizing, secondary and thermal electrons are well suited to trace single transitions on the surface of bulk material on the nanosecond‐micrometer scale. In contrast to other diagnostic probes the spatial resolution may be increased well below 1 μm by improved focussing of the primary electron beam.