Improved critical dimension control in 0.8-NA laser reticle writers

Modifications have been made to the ALTA(R) 3500 system to improve critical dimension (CD) control in three ways. First, the mean-to-target performance has been improved by increasing the repeatability of the measurement of optical efficiency, thereby more precisely setting the dose delivered to the photoresist. Second, the compressed dry air (CDA) used by the focus subsystem has been replaced with pressurized air drawn from the print head of the writing system. By using the humidity-controlled air from the print head, the water content of the photoresist is not affected by the focus subsystem. As a result, variability in CD uniformity that is dependent on some aspects of the pattern size and density are eliminated. Other pattern-dependent CD uniformity issues arising from process effects are also addressed. Finally, an option to allow the system to print with eight rather than four averaging passes has been evaluated. With an increased averaging of errors, improvement is seen in several performance parameters, particularly stripe butting, CD uniformity, and composite overlay.