DIAGNOSTICS AND REAL-TIME MONITORING OF PULSED LASER ABLATION

Abstract

Signal generation, diagnostics and real-time monitoring during pulsed laser ablation of solid materials are investigated. It is found that acoustic, optical and electric signals generated are closely related to laser processing conditions, material surface and bulk properties. A wide band microphone is used to record audible acoustic wave emission. For laser fluence less than the ablation threshold of solid substrate, the acoustic wave reduces gradually to zero with pulse number due to laser cleaning of surface contamination. For laser fluence above the ablation threshold, it also reduces with pulse number but to a stable waveform because of substrate ablation. The first peak-to-peak amplitude is selected as a characteristic parameter to estimate cleaning efficiency and ablation rate during the laser ablation. Optical emission spectrum analysis is carried out during the laser ablation of indium tin oxide (ITO) thin films for liquid crystal display (LCD) patterning, By evaluating when intensities of ITO spectral lines reduce to zero, the laser processing can be monitored in real time. By checking whether spectral lines for the substrate ablation appear, the processing without substrate damage can be fulfilled. Compared with other signal detection techniques, it is more applicable in the laser ablation of multi-element materials and multi-layer structures. Plasma emission spectrum and its dynamics are also analyzed during pulsed laser deposition of Ti thin film and laser deflash of molding compounds for integrated circuit (IC) packaging. To correlate with thin film quality, plasma plume is imaged by high-speed photography to investigate evolution of plasma sizes and kinetic energy during the pulsed laser deposition. An ultrafast phototube is applied to capture optical signals generated at early stage of laser ablation. There are two peaks in an optical signal with the first peak attributed to laser scattering and the second one to plasma generation. As laser fluence increases, the second peak rises earlier to overlap with the first one. Dependence of peak distributions on laser fluence and detection angle is analyzed. Time interval between plasma starting and scattered laser pulse termination is proposed as a quantitative parameter to characterize laser-plasma interaction. Plasma-induced electric field is measured by a tiny metal probe. Signal analyses show that it is resulted from the contribution of an electric dipole formed at the early stage of laser ablation. Formation of the electric field is briefly modeled. Signal variation with laser fluence, probe distance, substrate materials, laser wavelength and substrate bias is also studied. Based on the signal detection and analyses, relationships among signal characteristic parameters and laser processing conditions can be established to build different control databases. With a proper control loop, real-time monitoring of the laser ablation can be achieved.