ArF immersion lithography is essential to extend optical lithography. In this study, we characterized the immersion process on production wafers. Key lithographic manufacturing parameters, overlay, CD uniformity, depth of focus (DOF), optical proximity effects (OPE), and defects are reported. Similar device electrical performance between the immersion and the dry wafers assures electrical compatibility with immersion lithography. The yield results on 90-nm Static Random Access Memory (SRAM) chips confirm doubling of DOF by immersion as expected. Poly images of the 65-nm node from a 0.85NA immersion scanner are also shown.
The mechanism of focus latitude enhancement for contact/via hole printing is explained by approximating the axis intensity distribution of an image as a series of cosine functions to characterize the interference between each pair of diffraction beams. It is found that a phase-shifting mask ͑PSM͒ with symmetrical assist features improves the depth of focus ͑DOF͒ by introducing destructive interference to counterbalance the intensity fluctuation from constructive interference as defocus. A simple formula was derived to represent the capability of focus latitude enlargement. It shows that the extent of enhancement depends on the exposure wavelength and numerical aperture of a projection lens only. Increasing the degree of partial coherence degrades the focal range enlargement because a larger illumination angle elongates the destructive interference pattern in the optical-axis direction to weaken its ability for intensity compensation. On the other hand, the lack of constructive interference in dense hole imaging fails the mask pattern transfer, which limits the application of the phase-shifting method to pattern pitch greater than ͱ2/NA. A tiny amount of spherical aberration results in prominent asymmetrical defocus behavior because the wave deformation in the projection lens shifts the distribution of constructive and destructive interference patterns to opposite defocus directions. The printing characteristics of 0.17 m contact using an 18% transmission, rim-type attenuated phase-shifting mask are investigated to corroborate our analysis of defocus behavior. The dependence of depth of focus on pattern duty is stressed to elucidate the difference in mechanisms of focus latitude improvements for a sparse hole and periodic dense hole.
193-nm immersion lithography is the only choice for the 45-nm logical node at 120-nm half pitch and extendable to 32-and 22-nm nodes. The defect problem is one of the critical issues in immersion technology. In this paper, we provided a methodology to trace the defect source from optical microscope images to its SEM counterparts after exposure. An optimized exposure routing was also proposed to reduce printing defects. The average defect count was reduced from 19.7 to 4.8 ea/wafer.
This paper reports the water-leakage mechanism of the immersion hood in an immersion scanner. The proposed static analysis reveals the immersion hood design performance in defect distribution. A dynamic water-leakage model traces the leaked water and identifies its position on the wafer, during exposure. Comparing simulation to experimental results on bare-silicon and resist-coated wafers, the defect type, source of residuals, and critical settings on the immersion system were clearly identified. 1.INTRODUCTIONImmersion lithography is the only viable choice to produce 65-nm and 45-nm half-pitch circuits. Although 193-nm immersion lithography is being developed at an unprecedented pace, it still needs enormous efforts to meet the extremely tight requirements for the 45-nm node. 1,2,3 One of them is defect density. It needs to be as low as 1 defect per wafer pass, especially when many device layers are exposed with immersion scanners.The immersion hood (IH) is an important component of the immersion scanner for supplying, confining, and draining the water during exposure. Unfortunately, the fluid will leak from the IH due to insufficient constrain and the droplets left will produce watermark defects. Besides watermarks, the liquid coupling medium is more likely to carry particulates than the air coupling medium, no matter these particles are from materials, the wafer, or the exposure system itself. The particulates may be left through water leaking from the IH during wafer movement and cause printing and fall-on defects.Hence, preventing water leakage is one of the most critical issues for immersion defect reduction. This paper reports a static residuals analysis to monitor the IH design performance and define the correlation between airflow instability and water leakage in the IH. Furthermore, a dynamic water-leakage model has been setup, basing on the mechanism discussed in Section 3. This dynamic model traces the water-leakage trajectory during exposure routing.
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