Copper Calls for New Alignment Schemes
Aaron Hand, Managing Editor -- Semiconductor International, 7/1/2001
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Mukherjee-Roy, along with Rakesh Kumar of the IME and Ganesh Samudra of the National University of Singapore, evaluated overlay measurement target designs for copper dual-damascene processes, presenting results at the latest SPIE Microlithography conference in Santa Clara, Calif.
They evaluated a via-first approach to processing for copper with conventional dielectrics, and a trench-first approach for copper in low-k materials. They used a Nikon S203 step-and-scan system for the lithography process, and a KLA 5200 XP as the overlay measurement tool. They used a custom damascene reticle set designed by the Institute of Microelectronics in order to make use of its various overlay target designs.
For effective overlay metrology, it is important that the alignment marks from one chip level have high light scattering (providing high contrast) through to the next level, Mukherjee-Roy noted. For the via-first approach, via-to-trench alignments have low contrast with both conventional oxide and low-k materials. Although a thicker nitride etch stop layer could increase the contrast, it would be detrimental to the device speed. A via- or M2-to-M1 alignment could be done instead, but CMP could have adverse effects. Therefore, the researchers settled on a via-to-metal alignment scheme with a buried nitride hard mask for their evaluations. For low-k dielectrics, where a dual hard mask was used, relatively small via or trench step heights resulted in very low contrast through the oxide etch stop layer. But the team was able to improve contrast by using special overlay targets.
The researchers evaluated targets by their appearance, static and dynamic repeatability, target correlation, tool-induced shift, overall misregistration and residuals. They evaluated two types of overlay marks: trench in trench and wall in wall, both 1 and 2 µm thick.
For specs of £60 nm, a target correlation of >90% is desirable. For tool-induced shift (TIS), a low 3 s is desired because it cannot be compensated for. While aligning to a metal underlayer, 3 s increases significantly, Mukherjee-Roy said. In the studies, the 1 µm walls gave the worst TIS results, but the 2 µm walls performed better. For dynamic repeatability, the 2 µm trench targets proved to be the best.
Overall, trench in trench worked better than wall in wall did for the via-first approach with conventional dielectrics. In this case, the 2 µm trench targets performed best, giving better results for target correlation, dynamic repeatability and residual values. Wall in wall gave the best results (with good contrast) for organic low-k dielectric and copper, for which the dual hard mask scheme was used.
Company News
Numerical Technologies Inc. (San Jose) has announced an option for its Virtual Stepper system that enables more accurate inspection of complex photomasks. The Attenuated Module facilitates inspection of attenuated phase-shift masks (PSMs) based on silicon printability, thereby reducing the cost and turnaround time to develop subwavelength chips. Because of inspection and review challenges, verifying the quality of an attenuated PSM has traditionally been time- and cost-prohibitive. The module is expected to be available in the latest release of the KLA-Tencor PASS photomask inspection system and in equipment from other suppliers.
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Leica Microsystems (Wetzlar, Germany) is supplying WIN Semiconductors Corp. (Taiwan) with a Leica-ZBA electron-beam lithography tool, the third such installation in the region recently. WIN plans to use the system in production of its advanced 150 mm GaAs wafers.
Innolite AB (Stockholm, Sweden) announced that it has been approved by MEDEA+ to provide extreme ultraviolet (EUV) light source technology for the group's research and development efforts. A pan-European initiative, the MEDEA+ EUV source development consortium will also include Carl Zeiss, AIXUV GmbH, Alcatel, Thales Laser, Philips and Xtreme Technologies.
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