Allen Keith Bates

Long-term testing of optical components for 157-nm lithography

Photolithography utilizing 157-nm excimer lasers is a leading candidate technology for the post-193-nm generation. A key element required for successful insertion of this technology is the near-term performance and long-term reliability of the components of the optical train, including transparent bulk materials for lenses, optical coatings, photomask substrates, and pellicles. For instance, after 100 billion pulses at an incident fluence of 0.5 mJ/cm2/pulse optical materials, of which the primary candidate is calcium fluoride, should have an absorption coefficient of less than 0.002 cm-1, and antireflective layers should enable transmission of 98.5 percent for a two-sided coated substrate. Modified fused silica has emerged as a viable option as a transparent photomask substrate, and several approaches are being explored for transmissive membranes to be used as pellicles.

Long-term 193-nm laser-induced degradation of fused silica and calcium fluoride

We have completed a comprehensive evaluation of bulk materials designed for 193-nm lithographic applications. These studies are performed at realistic fluences and pulse counts in excess of 6 X 109. The outcome of the study shows that most calcium fluoride materials should meet the industry lifetime targets for use in lens applications. Some fused silica material also appears to meet lifetime expectations of the industry; however, large grade-to-grade variability in both absorption and laser-induced densification has been observed. We also report on the impact of transient absorption in fused silica on lithographic dose control.

Assessment of optical coatings for 193-nm lithography

We present an assessment of antireflective coatings for 193-nm lithography. Coatings from nine suppliers were exposed in a nitrogen ambient for up to 1.5 billion pulses at 15 mJ/cm2/pulse at 400 Hz. Sensitive metrology, developed for this study, included reflectance/transmittance measurements, in-situ ratiometric transmission measurements, and interferometric calorimetry for absorption measurements. The coatings from at least two suppliers withstood greater than 1 billion pulses with no observable degradation. Catastrophic damage observed on some samples included blistering and a dramatic transmission drop. Such damage occurred rather early (less than 100 million pulses).

Interactions between Servo Loops of an Optical Disk Drive

In optical disk drives, interactions between the electromechanical actuators and the servo error generation systems can cause instabilities in the operation of individual servo loops. We examine the effects of these interactions on the servo loops of an optical disk drive in which a galvano fine-tracking actuator, a push-pull tracking error detection scheme, and an astigmatic focus error detection method are used. Also, we describe a digital servo controller that effectively cancels the effects of the deleterious interactions, resulting in stable operation.