Picosecond-laser system for photomask repair with nanometer accuracy

摘要

A novel 351 nm picosecond-range pulsed, master-oscillator-power amplifier laser system, specially designed for an advanced mask repairing system LM700A, capable of repairing photomasks for 130 nm-design rule 1G DRAMs, has been developed. The front-end of the laser system is a diode-pumped, simultaneously active-mode-locked and Q-switched Nd:YLF laser and is capable of emitting short light pulses variable in the range between several ten- and several hundred picosecond. Extracted pulses from the mode-locked and Q-switched pulse trains are amplified by a double-pass amplifier and are subsequently frequency-converted to 351 nm by using LBO crystals for high-precision photomask repairing. Optimum irradiation conditions for opaque defect repairing have been investigated for by varying pulse duration to satisfy the stringent requirements such as for minimum repairing accuracy better than 30 nm, high transmission with minimum surface damage, minimum wall roll-up, etc. Mid-range pulses having around 200 ps have been found to be optimum to realize high quality repairing.