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Novel, At-design-rule, Via-to-metal Overlay Metrology for 193 nm Lithography Atsushi Ueno, Kouichirou Tsujita, Renesas Technology Corporation Hiroyuki Kurita, Yasuhisa Iwata, Mark Ghinovker, Elyakim Kassel, Mike Adel, KLA-Tencor Corporation
The effect of scanner aberrations pattern placement errors (PPE) in the copper interconnect lithography process is studied both in simulations and experimentally. New grating-based overlay targets on KLA-Tencor’s Archer AIM overlay metrology platform enable the measurement of device feature overlay. Studies undertaken at Renesas demonstrate that AIM targets exhibit superior performance over conventional box-in-box (BiB) targets. Comparison between CD SEM and optical overlay measurements show sensitivity to PPE. Good matching between AIM and device overlays was demonstrated.
As device features shrink beyond 90 nm, the discrepancy between conventional overlay measurement and device overlay has become a matter of concern. Resolution enhancement techniques such as non-conventional illumination and phase shift masks are in common use for 193 nm lithography processes. Such techniques significantly increase the impact of lens aberrations on pattern placement errors (PPE). Furthermore, the magnitude of these pattern placement errors is dependent on pitch and feature size. Brunner1 has simulated the effects of coma-aberration on the placement error of isolated lines, demonstrating placement errors of over 50 nm. Progler et al.2 have built a predictive device overlay model based on experimental and simulated results, taking into account the differential placement of device features and conventional overlay metrology structures. Further investigations of the lithographic effects on product patterns and test structures have been carried out by Bukofsky et al.3, 4 Wong5 has demonstrated the enhancement of PPE when using alternating phase-shift masks, as compared to conventional chromeon-glass masks. Tsujita et al.6 were studying the method to decrease lens aberration phenomena, and confirmed the effectiveness of negative tone lithography.
These investigations point to the yield risk associated with PPE, as well as to the necessity of overlay metrology at design rule feature sizes. Conventional box-in-box and frame-in-frame (we will use notation “BiB” for both types) overlay marks have lacked both the robustness and the information content when they have been segmented to design rule dimensions. Recently developed grating-based metrology marks from KLA-Tencor, based on AIM technology and offered on the Archer overlay metrology platform,7-9 have overcome these limitations. Optical simulations
Figure 1 is an example of optical simulation results for Simulated PPE Dependence on Feature Size for a 3rd Order Coma Aberration = 50 mλ
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pattern shift dependence on isolated feature size using the in-house aerial image simulator. The plot in this figure shows the predicted placement error caused by third order coma aberration. The baseline is pattern shift of 1 µm feature. Under the condition of annular (2/3) illumination, with enhanced coma aberration (50 mλ), the pattern shift dependence on feature size varying from 180 nm to 1200 nm is more than 5 nm. This shows that conventional BiB marks do not necessarily represent the real pattern shift of fine patterns in device. Figure 3. Left: “simultaneous” single layer AIM mark with 1 µm feature
Measuring PPE with the AIM mark
To investigate the potential yield related risks posed by PPE, and their impact on overlay metrology, it is essential to create and measure overlay marks whose feature size is at design rule. Such investigations, however, have been limited by existing metrology tool performance, because conventional BiB marks are not sufficiently robust, nor do they convey enough information when they are design rule-segmented. Compared to standard BiB, the new AIM gratingbased metrology mark shows superior precision, TIS 3 sigma and mark fidelity performance, and enables design rule segmentation (Figure 2). We proposed several design rule segmented marks, and applied them to the critical via to metal layers of a copper (Cu) damascene process. Optical overlay measurements were performed on the grating marks with different feature sizes and densities at different locations across the field of the exposure tool. The optical metrology study included two different types of grating marks: single layer “simultaneous” marks (Figure 3) and bi-layer overlay metrology marks Metal/Via: BiB vs. AIM 3
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Figure 4. Left: Bi-layer AIM mark with fine segmentation at design rule — 140/140 nm line/spaces on inner and 170/130 nm diameter/space vias on outer. Right: SEM image of fine segmentation.
(Figure 4). The former are ideal for characterizing feature size-dependent PPE and their variation across the field of the scanner slit under specific lithography illumination conditions. The latter type of marks are suitable as production overlay metrology marks, and they characterize the combined effect of PPE between two subsequent lithography steps. Overlay measurements of the simultaneous AIM mark (shown in Figure 3) reflects PPE difference between the solid (BiB-like) and design rule (device-like) features. Figure 5 shows an example of the measured PPE difference for various feature size and across-the-slit locations. Clearly, there is good qualitative matching to the simulation results (see Figure 1).
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Figure 2. Precision and TIS 3 sigma performance of BiB, non-segmented and design rule segmented AIM marks.
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on inner and isolated 180 nm line on outer. Right: SEM image of
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The overlay measurements of the design rule-segmented AIM targets across the slit demonstrate significant sensitivity to scanner aberrations. We have correlated the Archer AIM measurements with the CD SEM measurements of the device features. These CD SEM measurements
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Figure 5. PPE as measured by simultaneous marks, at different positions
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This procedure was repeated for each one of the nine lines in the grating, which improved the signal-to-noise ratio of the CD SEM measurements. Thus, the uncertainty of the CD SEM measurements performed in this approach was reduced roughly by a factor of √9 = 3. Another important advantage of this approach is that the same object was measured both optically (by Archer AIM) and by means of a CD SEM. On the other hand, the shortcoming of this approach is that, because of the need to compare between two complementary CD SEM images, this method is sensitive to the image rotation due to poor focus repeatability. The results of this approach are shown in Figure 7.
across scanner slit (in mm), as a function of isolated feature size (in nm), with enhanced coma aberration (40 mλ). Baseline is “large” (BiB-like) 8
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Measurements on the AIM marks were done in the following way. CD SEM images were analyzed and then selected from the boundary between the inner and outer parts of the AIM mark (Figure 3, image on the right is an example). This image depicts one out of ten junctions between the outer (thin, top) and inner (thick, bottom) lines from the top right quarter of the AIM mark. For every such image there exists a complementary image from the bottom left quarter of the AIM mark so that, by design, the centers of symmetry of both thick and thin lines coincide (Figure 6). The edges L, R, L1, R1 and their counterparts L’, R’, L1’, and R1’ were detected and the centers of symmetry of the inner and outer gratings were found. The misregistration between the centers of symmetry determines the “overlay” (which, in the case of the simultaneous AIM mark, is the PPE difference between the BiB-like inner and device-like outer patterns).
Figure 6. CD SEM overlay measurements on the AIM mark.
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Figure 7. Variation across scanner slit of PPE between large 1 micron feature and 180 nm line, as measured by simultaneous marks and validated by CD SEM.
An alternative approach was used for CD SEM overlay measurements on the special device-representing CD SEM marks shown in Figure 8. Two CD measurements (“A” and “B”) were performed, and the overlay was calculated as OVL=(A+B)/2 - 0.6 µm, where 0.6 mm was the distance between the centers of
Figure 8. CD SEM measurements on a specially designed device-representing mark.
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Direct comparison between the optical (Archer AIM) and device-like (CD SEM based) overlay measurements was made. Two different approaches for the CD SEM overlay measurements were used. Both of them indicated good correlation between the Archer AIM and device overlay.
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References
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1. T. A. Brunner, “Impact of lens aberrations on optical lithography,” IBM J. Research & Development, vol. 41, 1997. 2. C. Progler, S. Bukofsky, and D. Wheeler, “Method to budget and optimize total device overlay,” in Optical Microlithography XII, Luc Van den hove, Editor, Proceedings of SPIE Vol. 3679, 193-207 (1999). 3. S. Bukofsky, and C. Progler, “Interaction of pattern orientation and lens quality on CD and overlay errors,” Optical Microlithography XIII, Christopher J. Progler, Editor, Proceedings of SPIE Vol. 4000, 315-325 (2000). 4. S. Bukofsky, and C. Progler, “Product-like aberration test structures,” Proceedings of Interface 2000 Conference, 121-133 (2000). 5. A. K. Wong, Resolution enhancement techniques in optical lithography, Bellingham, SPIE Press, 2001, Ch. 5, pp. 126-128. 6. K. Tsujita, Y. Yamauchi, and A. Ueno, “Effect of negativetone mask llithography on lens aberration phenomena”, Proc. SPIE Vol.3679, p.382-393 (1999) 7. M. Adel, M. Ghinovker, B. Golovanevsky, P. Izikson, E. Kassel, D. Yaffe, F. Bruckstein, R. Goldenberg, Y. Rubner, and M. Rudzsky, “Optimized Overlay Metrology Fiducials: Theory and Experiment”. To be published in IEEE Transactions on Semiconductor Manufacturing. 8. M. Adel, M. Ghinovker, J. Poplawski, E. Kassel, P. Izikson, I. Pollentier, P. Leray, and D. Laidler, “Characterization of Overlay Mark Fidelity”. Proc. SPIE Vol. 5038, p. 437-444 (2003) 9. M. Adel, M. Ghinovker, J. C. Robinson, E. Kassel, D. C. Benoit, G. S. Seligman, and J. A. Allgair, “Performance study of new segmented overlay marks for advanced wafer processing”, Proc. SPIE Vol. 5038, p. 453-463 (2003).
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Figure 9. Variation across scanner slit of overlay between via and metal processed layers, as measured by design rule segmented AIM marks and validated by CD SEM.
the hole and the line by design. These CD SEM overlay measurements were compared with the Archer AIM measurements done on the adjacent AIM mark with similar inner (isolated 180 nm metal lines) and outer (two chains of isolated 170 nm via holes) structures. The results are shown in Figure 9. The advantage of this approach is its reduced sensitivity to the image rotation and easy automation of the measurement procedure. The shortcoming is poorer signalto-noise ratio and, therefore, increased uncertainty of the CD SEM measurements. As observed in Figure 9, overlay variations across the slit due to scanner aberrations are as large as 10 nm peak to valley. However, the AIM measurements typically match CD SEM measurements within 2-3 nm. Conclusions
We conclude that, for copper interconnect, direct overlay metrology of design rule features is achieved with design rule-segmented grating overlay marks. This enables feedback of device overlay information which, in turn, allows more accurate overlay control, thus decreasing the risk of rework or potential yield loss.
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