Mechanisms of Excimer Laser Ablation of Wide Band-Gap Materials: the Role of Defects in Single Crystal MgO

ABSTRACTLaser ablation has important applications in surface modification, materials analysis, and thin film deposition. We have been examining the details of processes that lead to the emission and formation of particles (atomic/molecular ground state neutrals, excited neutrals, tions, electrons) when wide band gap materials are irradiated with pulsed UV laser light. Etching and deposition of wide bandgap materials is of particular interest due to their excellent insulating and optical properties. Our studies bear directly on achieving control of emission intensities and particle characteristics for use in film deposition and materials analysis. In model wide bandgap materials such as single crystal alkali halides and MgO (nominally transparent materials), exposure to repeated pulses of 248 nm excimer laser radiation of a few J/cm2 results in substantial interaction including extensive biaxial deformation and cleavage. Significant surface heating also occurs, consistent with strong free-carrier/laser interactions. We present strong evidence that achieving intense emission of atomic, molecular, and ionic particles actually depends on point defect production by laser-induced deformation and fracture. Defect production via dislocation motion yields orders of magnitude increases in laser vaporization of these wide bandgap materials, including cluster ion formation. The dependence of the laser-material interaction on dislocation density and mobility, as well as point defect density, suggests several novel strategies for the enhancing the ablative response or preventing laser damage.