ARO coating passes 1 billion pulse mark at MIT

10/01/1998

ARO coating passes 1 billion pulse mark at MIT

Researchers at MIT Lincoln Laboratory, Lexington, MA, have reached a milestone in durability testing for 193-nm lithography applications with a coating from Alpine Research Optics (ARO), Boulder, CO. Specifically, an ARO "high reflector" coating has been exposed to greater than 1 billion pulses at a fluence of 15 mJ/cm2/pulse - within the range of concern - without showing any significant decrease in reflectivity. Previously, one of ARO`s antireflection-coated components reached 1.2 billion pulses under the same conditions.

This work on lifetime testing of optics for microlithography systems at 193 nm is lead by Mordechai Rothschild under funding from SEMATECH, Austin, TX. The experimental setup irradiates samples at 400 Hz (35 million pulses/day) in a nitrogen purged ambient. Reflectance changes are monitored ex situ with a UV-visible spectrometer (190-600 nm).

Optical coatings represent an integral part of 193-nm exposure systems because they are applied to virtually every surface for antireflective, reflective, or beam splitting functions. Ideally, the coatings in projection optics should not degrade for 10 years, which corresponds to 100 billion pulses at 1 mJ/cm2/pulse. Common belief has been that laser damage to optics may take place at moderate fluences - 10-20 mJ/cm2/pulse - and pulse counts over 200 million.

In a separate test at Lincoln Lab, optics are being exposed to lower fluences. Here, an antireflection-coated optical component from ARO has already accumulated over 540 million pulses at 4 mJ/cm2 (as of August 1998), with no detectable change in performance. The group at MIT will continue to irradiate this component in order to characterize its lifetime, and is working to determine the effect of exposure fluence on performance. - P.B.