MECHANISM OF PICOSECOND ULTRAVIOLET-LASER SPUTTERING OF SAPPHIRE AT 266 NM

Ultraviolet laser sputtering of sapphire was observed to result in droplet ejection when done in air, pressurized air, or in 20 mTorr pressure. Such droplets were collected and studied with scanning electron microscopy and optical microscopy. The collected particles were spherical and ranged in size up to 3 μm in diameter. Some particles appeared to have been molten and splattered on the collection plate. The size of the particles was independent of the ambient pressure. However, the collection efficiency of the collection plate was ∼30 times greater at 20 mTorr pressure as compared to one atmosphere. The similar result for measurements of the sputter depth under a range of background pressure indicates that the mechanism for the laser sputtering of sapphire at 266 nm with 30 ps pulses and a fluence near 10 J/cm2 is independent of the ambient pressure. Redeposition of debris onto the sputtered sample was significantly greater in one atmosphere pressure than in reduced pressure. These findings indicate that hydrodynamic sputtering is the most likely mechanism for the laser sputtering of sapphire at high sputter rates.