Laser backside etching of fused silica due to carbon layer ablation

The backside ablation of a absorbing carbon layer onto fused silica is studied in air and water confinement in comparison. The confinement influences the etch rate and the laser fluence dependence of the etch rate significantly while the threshold fluence is almost the same. The different confinement of the laser induced plasma results in the observed rate saturation in the case of air and in a linear growing rate in the case of water confinement at medium laser fluences. The less dense air confinement permits a faster plasma expansion of the laser plume than in the case of water confinement and effects consequently the interaction time and interaction strength of the laser plume with the fused silica surface. The differences in the laser-plasma-substrate interaction cause the observed rate saturation at weak interaction (air) and the growing etch rate at strong interaction (water). Thus, the confinement situation controls the interaction process in the case of backside ablation and should be considered in indirect material processing methods such as LIBWE and LESAL, too. PACS 81.65.C; 81.05.K; 79.20.D; 61.80.B; 42.55.L; 68.45.D