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Q‐switched infrared, green, UV and DUV lasers are being used routinely for various microelectronics and photovoltaic manufacturing applications, such as ablation and cutting of dielectric materials, LED scribe and cleave processes in microelectronics, and scribing thin film solar cells. To create functional good results when micromachining any material, the laser energy density needs to be set higher than the material removal threshold and a reasonable laser beam spot overlap is required. The maximum scribing speed achieved using a system with a pulsed laser source therefore is usually limited by the laser repetition rate and the energy per pulse available at that repetition rate. For a given repetition rate, as processing speed increases the beam spot overlap decreases and at certain critical processing speed laser beam spots gets separated in space and continuous scribing of material is not possible. This fundamental limitation has to be solved in order to achieve faster processing speeds, higher throughput, and lower cost per part. To overcome this limitation in next generation laser processing systems, SpectraPhysics has developed and evaluated high repetition rate mode‐locked lasers as an alternative to Q‐switched lasers. Using a high average power mode‐locked laser operating at 80 MHz, we have been able to demonstrate processing speeds that are an order of magnitude higher in thin layers of select materials than that with current Q‐switched technology. The following article reviews results obtained for micromachining dielectrics, scribing and cleaving blue LEDs, and scribing different solar cell materials.

Based on experimental results, a qualitative process model for reactive ablation of ferrous materials is suggested, which includes the absorption of the radiation to the workpiece, oxidation of the workpiece material, formation of a chip or wedge and removal of the oxide by bending perpendicular to the workpiece. The superior role is ascribed to the exothermal reaction of the matrix material by increasing the absorption and thus initiating a stationary process of either chip or wedge formation. The model is verified by different experiments, comparing the ablation results of steels with varying carbon content and therefore different heat conductivities, the importance to the process of which becomes obvious in the phenomenon of the sudden transition from chip removal to wedge removal. The influence of impurities on the formation of the chip is explained as well as the mechanism of chip bending.

The phenomenon of laser supported absorption (LSA) of CO2 laser radiation is investigated with spatially and temporally resolved plasma spectroscopy in Argon as ambient gas at pressures of 1 -iO Pa. A TB-CO2 laser with a gas mixture of C021N2/He = 114/4 at a total pressure of half an atmosphere is used. The pulse length of the Th-C02 Laser is about 6 ts at a mean power of some 100 kW. LSA-phenomena and material ablation processes are compared with that of a con ventional TEA-CO2 short pulse laser. It could be shown that longer pulses of less mean power make material ablation more efficient ifthe absorption wave becomes transparent within the pulse time ofthe laser. Maximum electron density of 2.5 .1017 and a temperature of 3 eV were measured 1.5 is after the begin of the irradiation for an expanding aluminium plasma into vacuum. 1.ThE LASER SYSTEMBy using a non conventional gas mixture and total pressures of about half an atmosphere in a modified TEA-C02-Laser, see fig. 1, pulse-durations in the range of some hundred nanoseconds to 10 p.s could be produced. As the pulse-length we define the time within which 85 % ofthe total pulse energy has been emiued by the laser. On the one hand the CO2 molecule is excited directly by electron impact and on the other by energy transfer from the excited nitrogen molecule. The rate ofenergy transfer' is 1.4 x iO mbar1 s .The laser pulse consists of a 50 -100 ns peak of high amplitude corresponding to the direct excitation mechanism and a 1 -10 ts part of lower amplitude due to the energy transfer from the nitrogen molecule. 1) By lowering the partial pressure of CO2 as well as the total pressure the pulse length is prolonged, the laser efficiency however decreases. The total pulse energy amounts 500 mJ to 3J at pulse lengths of 10 ts to some 100 ns, respectively, see fig 2. A stable resonator is used, consisting of a gold coated copper mirror with 10 m focal length and a plane ZnSe output coupler with a reflectivity of 60 %. In multimode operation the measured beam divergence was approximately 3 mrad. At a CO2 partial pressure of less than ca. 25.102 Pa (about 8 % of the total pressure) the threshold gain condition is no longer fuffilled. For pulse durations exceeding 10 psthere must be used an output coupler with higher reflectivity or a folded resonator.2'3) The efficiency ofthe laser would be much lower then. The electrical circuit is a simple charge transfer circuit. The capacity of the main discharge capacitor bank is 60 uP. The switch element is a pseudo spark switch.4 IGNITION OF A PLASMA AND ABSORPTION WAVESInvestigations were made on the plasma formation and absorption waves. In air there is a plasma generated at irradiation with pulsed CO1 lasers above an threshold power density of W/cm2 which decreases in the vicinity of surfaces to about i07 W/cm"6. From this initial plasma a laser supported absorption wave (LSAW) starts and moves towards the incoming laser beam. As the duration of the laser pulse is in our case one order of magnitude longer than that of the TEA CO ...