Showing papers by "Minghui Hong published in 1999"

Electric signal detection at the early stage of laser ablation in air

Abstract: A tiny metal probe was used to detect electric signals induced at the early stage of laser ablation in air. It is found that the electric signals result from probe ablation, plasma–probe interaction, and plasma-induced electric field. The recorded signals strongly depend on the probe positions. For a probe placed out of the plasma–probe interaction region, the detected electric signal is a negative peak in the nanosecond range, due to the plasma-induced electric field. The peak arrival time corresponds to the total amount of ion emission from the substrate surface, and therefore, does not vary with the probe position. The signal amplitude is inversely proportional to the square of the probe distance, consistent with the distance dependence of the field intensity from an electric dipole. The signal amplitude increases with the laser fluence while the peak arrival time reduces, reflecting the earlier plasma generation at a higher laser fluence. Both peak width and its arrival time of the electric signals increase with laser fluence and tend to saturate above 6.4 J/cm2. The electric signals were analyzed for laser ablation of different substrate materials. The electric signal detection was also applied to monitor the laser cleaning of organic contamination in real time. The mechanism of the electric signal generation and the process of electron and ion emission are briefly discussed.

Laser plasma interaction at an early stage of laser ablation

Abstract: Laser scattering and its interaction with plasma during KrF excimer laser ablation of silicon are investigated by ultrafast phototube detection. There are two peaks in an optical signal with the first peak attributed to laser scattering and the second one to plasma generation. For laser fluence above 5.8 J/cm2, the second peak rises earlier to overlap with the first one. The optical signal is fitted by a pulse distribution for the scattered laser light and a drifted Maxwell–Boltzmann distribution with a center-of-mass velocity for the plasma. Peak amplitude and its arrival time, full width at half maximum (FWHM), starting time, and termination time of the profiles are studied for different laser fluences and detection angles. Laser pulse is scattered from both the substrate and the plasma with the latter part as a dominant factor during the laser ablation. Peak amplitude of the scattered laser signal increases but its FWHM decreases with the laser fluence. Angular distribution of the peak amplitude can be fitted with cosn θ(n=4) while the detection angle has no obvious influence on the FWHM. In addition, FWHM and peak amplitude of plasma signal increase with the laser fluence. However, starting time and peak arrival time of plasma signal reduce with the laser fluence. The time interval between plasma starting and scattered laser pulse termination is proposed as a quantitative parameter to characterize laser plasma interaction. Threshold fluence for the interaction is estimated to be 3.5 J/cm2. For laser fluence above 12.6 J/cm2, the plasma and scattered laser pulse distributions tend to saturate.

Plasma diagnostics at early stage of laser ablation

Abstract: Plasma diagnostics by optical and electric signal detection at the early stage of laser ablation have been investigated. An ultrafast phototube was applied to capture the optical signal. There are two peaks in the optical signal, with the first peak attributed to laser scattering and the second one to plasma generation. As the laser fluence increases, the second peak appears earlier to overlap with the first one. The dependence of the peak distributions on the laser fluence was analyzed. The time interval between the plasma starting and the end of the laser pulse is proposed as a quantitative parameter for characterizing the laser–plasma interaction. A tiny metal probe was used to record the electrical signal, with two negative peaks detected. The first peak has a duration of about 50 ns and its maximum amplitude position does not change with probe distance. It is attributed to a plasma-induced electric field. The field results from an electric dipole due to charge separation in the early stages of laser ablation. Variation of the first peak profile with probe distance and substrate bias was also studied.

Optical detection of laser plasma interaction during laser ablation

Abstract: Laser plasma interaction during pulsed laser ablation is investigated by ultrafast phototube detection. 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. The signal is fitted by different distribution functions for the laser scattering and the plasma generation. Peak amplitude and its arrival time, full width at half maximum (FWHM), starting time and termination time of the distributions are studied for different laser fluences and detection angles. Laser pulse is mainly scattered from the plasma during the laser ablation. Peak amplitude of the laser scattering increases but its FWHM decreases with laser fluence. Angular distribution of the peak amplitude can be fitted with cos n θ (n=4) while detection angle has no obvious influence on the FWHM. In addition, FWHM and peak amplitude of the plasma increase with laser fluence. However, its starting time and peak arrival time reduce with laser fluence. Time interval between plasma starting and scattered laser pulse termination is proposed as a quantitative parameter to characterize laser plasma interaction. Threshold fluence for the interaction can be estimated to be 3.5 J/cm 2 for KrF excimer laser ablation of silicon. For laser fluence above 12.6 J/cm 2 , the plasma and scattered laser pulse distributions tend to saturate.

Laser texturing processes by reflected light detection

Abstract: Laser texturing technique has been established to provide low flying height and low stiction required for manufacturing high storage density media. The characteristics of the laser bumps can be precisely controlled, and are critically important for the excellent tribological performance. In the study, the hard disks have been textured successfully using the argon ion laser with the aid of an acoustic-optic modular in the optical path. Alternative laser bumps can be formed with various bump shape and bump height. The topography of the laser bumps are observed using AFM. Laser bumps are formed because of the modification of laser beam on the substrate during the heating and cooling processes. In attempt to study the bump formation mechanisms, a photodiode was employed to detect the reflected and scattered laser light, which irradiates on the hard disk surface to form laser bumps. The detected signals were studied under various laser power and pulse duration. It was found that there is a good correlation between the detected signal and the laser bump characteristics. The system has been proved to be an effective and convenient method to study the laser bump formation processes, and to in situ diagnose the laser bump characteristics.