AVS 50th International Symposium
    Surface Science Tuesday Sessions
       Session SS-TuP

Paper SS-TuP14
Relating Polarization to Optical Absorption and Ablation of Silicon/Silicon Oxide Surfaces

Tuesday, November 4, 2003, 5:30 pm, Room Hall A-C

Session: Poster Session
Presenter: S.M. Yalisove, University of Michigan, Ann Arbor
Authors: Y.N. Picard, University of Michigan, Ann Arbor
H. Liu, University of Michigan, Ann Arbor
J.C. Pentland, University of Michigan, Ann Arbor
J.P. MacDonald, University of Michigan, Ann Arbor
J. Nees, University of Michigan, Ann Arbor
G. Mourou, University of Michigan, Ann Arbor
S.M. Yalisove, University of Michigan, Ann Arbor
Correspondent: Click to Email
A significant benefit of machining materials using lasers with pulse lengths on the order of femtoseconds, rather than nanoseconds or picoseconds, is the reduced size of the damage region associated with the machined areas. However, depth of absorption, absorption mechanisms, and nano-scaled modifications of the near surface are still not fully understood and characterized when machining at or just below the ablation theshold of the material using femtosecond lasers. The direction of the laser electric-field vector relative to the sample surface is estimated to be directly proportional to the magnitude of optical absorption, and as a consequence, the ablation threhold of the material. Our recent studies have demonstrated a factor of 3 difference in the estimated ablation threshold for S versus P polarization (referenced to the sample surface plane) when machining silicon at grazing incidence (86°) in air. We extend these studies to grazing incidence machining of silicon with no native oxide on the surface as well as silicon with different oxide thickness, all under vacuum. We use a Ti:sapphire laser with 800 nm wavelength, 120 femtosecond pulses to irradiate the surface at both S and P polarizations. The intensity of the laser irradiation ranges from below to above the ablation threshold for silicon (~200mJ/cm@super 2@). We present results from clean Si (native oxide removed by HF etching or Shiraki method) as well as those from Si with a native oxide and thermally grown oxide layers. Samples are machined inside a vacuum chamber capable of reaching a pressure of 10@super -11@ Torr. For silicon samples with no oxide present on the surface, surface structure during irradiation is monitored using reflected high energy electron diffraction (RHEED). SEM and AFM results show differences in the size and morphology of the damage region, which are then correlated to the ablation threshold.