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Nd:glass slab laser for x-ray lithography

A Nd:Glass laser has been used to generate laser produced plasmas (LLP). The plasma emission in the keV range useful for X-ray lithography has been measured. Lithography with sub-micron linewidths has been demonstrated with a thin absorbing mask. The Nd:Glass slab laser is operated in a Q-switched one-dimensional unstable resonator. The resulting beam quality is a few times diffraction limited and is focused to an area less than the lO cm2 in our vacuum chamber. We have operated at 5 J up to 4 Hz repetition rate without any degradation of the laser output. Injection mode locking of the Nd:Glass laser with 0.7 ns pulses increases the intensity of the Q-switched laser output by about a factor of 10 and allows us to achieve a total integrated pulse length of less than 10 ns. Optical damage limits the laser intensity. The damage threshold for injection mode locked pulses focused into a Nd:Glass slab outside the laser cavity is about 20 J/cm2. However, we have observed another damage mechanism at lower intensities in Nd:Glass slabs in use in the laser head. Brown discoloration occurs in filaments along the laser beam path and we believe solarization with the help of self-focusing and the ultra- violet flashlamp radiation may be occuring. We are still investigating this phenomenon but at present it is limiting the laser output to only 2 J per pulse. The focused laser intensity is 2.1013 W/cm2 on a solid copper target in our vacuum chamber. The plasma emission in the keV X-ray range has been measured through a variety of thin film X-ray filters with a Hamamatsu micro-channel plate detector. Using the published values for the detector quantum efficiency, the micro-channel plate gain, and the filter's transmission spectra, we estimate that the conversion efficiency in the plasma from laser radiation to soft X-rays of energy greater than 0.5 keV is around 2%. We have performed single-level demonstration exposures of PNMA resist through a 10 im thick aluminised Kapton debris shield and a 4 im thick Boron Nitride X-ray mask supplied by Piero Pianetta the the Stanford Synchroton Research Laboratory. The 1.0 pm linewidth gold absorber patterns on the mask are accurately reproduced in the resist. The PMMA resist exposure rate at a 5 cm working distance from the plasma has been measured as 0.2 micron per lO J of total laser energy so we obtain a single layer exposure in about 30 minutes. The potential for improvement is enormous. The X-ray signal measured through the mask increases exponentially with the laser energy on target so increasing the focused intensity will reduce exposure rates dramatically. Available improvements in higher average power lasers and sensitised resists both offer an order of magnitude improvement over this system . These results indicate excellent potential for commercial LPP X-ray lithography.