The Nikon eReview

Immersion lithography has transitioned from R&D to production application in record time, with Nikon immersion scanners being used in high-volume manufacturing worldwide. Reporting on the current status of immersion for NAND and CMOS devices at LithoVision 2008, Soichi Inoue, Group Manager of Toshiba Corporation Lithography Process Development, announced that immersion lithography was sufficiently mature for 45 nm half pitch, with Toshiba already producing 43 nm HP NAND flash devices using the Nikon NSR-S610C.

Presentations by Katsushi Nakano, System Engineer of Nikon Corporation, and Inoue at the LithoVision symposium in February focused on immersion technology advancements and progress in manufacturing applications using the Nikon NSR-S610C. The S610C uses a multi-axial catadioptric lens to deliver a 1.30 NA for high-volume applications at 45 nm and beyond. The lens was designed to minimize flare and improve aberration control, while the Nikon Tandem Stage enables frequent focus and illumination calibration to enhance CD uniformity and stability. In addition, the Infrared Aberration Control (IAC) system reduces RET-induced thermal aberrations that can occur when using dipole or extreme advanced illumination strategies.

Inoue presented data demonstrating a 400 nm depth of focus with more than 8% exposure dose latitude, and delta CD < 1.9 nm (3 sigma) for 43 nm L/S using the S610C with crosspole polarized illumination. Nakano presented autofocus (AF) data from the S610C with AF accuracy of 15 nm (3 sigma) for complete shots and a mere 18 nm for all shots on the wafer – made possible by the Nikon Tandem Stage calibration capabilities. Enhanced imaging performance was the main driver for immersion lithography, and clearly Nikon technology is satisfying that key objective.

In volume manufacturing, overlay accuracy and stability are vital factors for immersion lithography’s success. The Tandem Stage reduces variability by processing wafers on the single exposure stage—avoiding stage mismatching errors and enhancing overlay accuracy with frequent calibrations. Additionally, Local Fill Technology uses only surface tension for water containment, which eliminates wet-to-dry overlay matching issues. As a result, excellent immersion overlay is being achieved in manufacturing today. Inoue confirmed that S610C overlay performance satisfies the requirements for 40-45 nm half pitch processing, and presented single machine overlay data from the S610C averaging less than 5 nm (3 sigma). Nakano also shared twelve wafer overlay data with 3 sigma X=4.2 nm and Y=2.9. These champion data actually exceed the requirements for 45 nm mass production, and are very close to achieving the increasingly challenging requirements for double patterning.

Early on, defectivity had been considered a potential showstopper for immersion lithography. However, Nikon Local Fill Technology with no air curtain enables defectivity levels equivalent to today’s dry scanners. Nikon also partnered closely with resist suppliers to deliver systems fully compatible with a variety of resists, both with and without topcoats, using a range of receding contact angles and no restriction on advancing contact angle. Nakano shared multi-wafer data, processed at full throughput (500 mm/sec), using a very high contact angle, and free of bubbles or watermarks—that validated the ability of the S610C to deliver maximum throughput (> 130 WPH) with no immersion defects, and the processing flexibility needed in high-volume manufacturing.

Immersion lithography has clearly come a long way in a very short time. This relatively new technology has already successfully demonstrated 45 nm manufacturing readiness, with customers such as Toshiba generating devices below 45 nm half pitch using the Nikon NSR-S610C.

Toshiba Using NSR-S610C Immersion Scanners for 45 nm Manufacturing