Magazine spring04 novel design rule

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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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ISO Feature Size [nm] Figure 1. PPE, determined by simulation, as a function of isolated feature size (in nm), for annular (2/3) illumination with enhanced coma aberration (50 mλ). Baseline is “1 µm” feature overlay.

Spring 2004

www.kla-tencor.com/magazine

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