Managing MEF

Ruth DeJule, Associate Editor -- Semiconductor International, 7/1/1999

In 1995, researchers at Toshiba and IBM first defined Mask CD deviation Enhancement Factor (MEF) to quantify the effect of mask CD errors on lithographic performance. MEF is the slope of the reticle linewidth versus resist linewidth curve, indicating the transfer of CD errors from the reticle to the wafer. A MEF value greater than 1 indicates a mask CD deviation is magnified on the wafer. For 4X lithography, a 40 nm mask error and a MEF of 2 will transfer a 20 nm printed error onto the wafer, instead of a 10 nm error (see Figure). The culprit is sub-wavelength lithography where the features imaged are smaller than the illuminator wavelength. This occurs at 0.18 µm design rules and k1 values below 0.5. While MEF is unity for large features, it increases rapidly when CDs are less than 0.5 l/NA for line-space patterns and 0.75 l/NA for contacts.

Select reticle enhancement techniques (RET) and a change in stepper reduction ratios are MEF abatement solutions. Clearly, slightly larger reduction ratios on steppers to 6X or even 8X would provide greater freedom from tighter specifications. But while this is straightforward, the expense of new lithography tools may make RET a more attractive approach. For example, certain kinds of phase-shift masks (PSM), such as dark field alternating PSMs adopted by Numerical Technologies, reduce MEF. In this case, a narrow chrome line separates a pair of clear rectangles; one phase-shifted 180° relative to the other. Studies indicate progressively smaller chrome lines are less sensitive to errors. The opposite is true for binary masks where smaller chrome gaps lead to larger MEF. But not all RETs reduce MEF. The sub-resolution features used in optical proximity correction (OPC), for example, add to the complexity of the lithographic process and magnify reticle CD errors in the printed image.

Fig. 1 For a MEF>1, a small CD error on a 4x mask can transfer the same magnitude of error onto the wafer. (Source: KLA-Tencor)

Since MEF begins with errors on the reticle, tighter CD control and metrology and tighter enforcement of defect inspection parameters are required. Characterization must be performed in each area across the reticle as well as the patterned wafer because MEF is not uniform (isolated features generally have a lower MEF than dense features) and defects near the feature edge are magnified by MEF as are CD errors. This creates a paradigm shift in sample size, applications of traditional wafer metrologies to reticles and new developments in defect inspection tools, according to Scott Ashkenaz, VP of marketing for lithography module solutions at KLA-Tencor. Typically, reticles are specified with 5 to 10 measurement sites using white light or i-line metrology. However, with tighter specifications in both CD and defects, the 1 µm optical metrology limit and small sample sizes are no longer adequate. Generally used at the wafer level, CD SEMs, with resolutions ~4 nm, are now being applied to mask sets manufactured for 0.15 µm memory cells having CDs of 0.6. Initially, CD SEM on quartz reticles posed charging problems; however, electronics and software have been developed to minimize changing effects on the surface of the mask, Ashkenaz said. Even with the higher resolutions and 5X to 15X more sites, overall measurement times remain about the same, ~5 min/mask.

In the same vein, defect inspection systems have gone to shorter wavelengths in response to MEF. Typically, inspection systems employ blue light sources with numerical apertures (NA) less than 0.7. Now, UV defect inspection systems have been developed that scan the reticle and analyze every printed geometry, detecting defects as small as 150 nm, 25% smaller than blue light. For detection of defects as small as 120 nm, a new UV system, soon to be introduced by KLA-Tencor, has a NA>0.7. The importance of MEF will be reflected in the next iteration of the SIA Roadmap where a proposal has been made for dividing mask-making specifications by MEF. Managing MEF begins with knowing the MEF for your process.