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A production-compatible method for the correction of image-placement (IP) error over a 1x stencil mask as used for proximity electron lithography (PEL) has been demonstrated. The mask IP error as measured using a newly developed metrology tool was fed forward to the PEL stepper, LEEPL-3000 and corrected for via the fine deflection of the electron beam. The overlay errors with respect to the substrate patterned by the ArF scanner have decreased from 63.6/59.3 nm to 26.1/36.4 nm in the x/y directions, but they are still larger than the errors of 15.2/14.8 nm for the conventional feedback method. Therefore, some improvements in the metrology method, the mask chucking method, the mask flatness and so on are required.

Proximity electron lithography (PEL) using the ultra-thin tri-layer resist system has been successfully integrated in our dual-damascene Cu/low-k interconnects technology for the 90-nm node. Critical comparison between conventional ArF lithography and PEL as to the via-chain yield for test element groups (TEGs) including approximately 2.9 million via chains was performed to demonstrate its production feasibility.

The performance of the LEEPL production tool is discussed from the framework of the litho-and-mask concurrent development schemes to establish the feasibility of proximity electron lithography (PEL) especially for contact and via layers in the 65-nm technology node. The critical-dimension (CD) uniformity of 4.7 nm has been achieved for 90-nm contact holes over the 1x stencil mask. Thus, the mask patterns can be transferred onto the resist layer with CD errors of less than 10%, even if the mask-error enhancement factor (MEEF) of 1.6 is taken into account. The mask manufacturability is improved if the MEEF further decreases via the use of thinner resists. Meanwhile, the overlay accuracy of 21.1 nm has been achieved in mix-and-match with the ArF scanner, with the intra-field error of only 5.1 nm owing to the real-time correction for the mask distortion. Also, the conditions for splitting dense lines into two complementary portions have been determined to avoid the pattern collapse in wet-cleaning and drying processes. The critical length of 2 µm is fairly safe for 70-nm lines if the low-damage drying is employed. The inspection tool based on transmission electron images cannot detect all printable defects without further optimization, hence a future challenge.

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