Approach to full-chip simulation and correction of stencil mask distortion for proximity electron lithographyWe have devised a new mask format for low-energy electron-beam proximity-projection lithography ͑LEEPL͒ that enables faster and more accurate image transfer from the mask to a wafer, in comparison with the single-membrane mask as originally proposed. Simultaneous exposure over four adjacent complementary quadrants of the mask, synchronized with step-and-repeat motion of the wafer stage, yields one complete pattern with the throughput of ϳ30 wafers/h. Mechanical analysis has demonstrated that the new format also offers better controllability of image placement owing to smaller membranes, even though the gravitational deformation of the mask, for example, must still be corrected for.

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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.