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The technique for improving optical projection-system resolution by phase-shifting alternate apertures of a periodic grating was introduced in 1982. This halves the frequency content of the image passing through the optics and should therefore double the effective resolution of such patterns. Unfortunately, as feature separation increases, the efficacy of this method diminishes. Previous work applying a similar approach to isolated features involves introducing minute, non-printable, phase-shifted assist slots around the desired feature. The diffraction side-lobes of these slots constructively interfere with the center lobe of the primary aperture. The resolution enhancement afforded be this technique is limited by the printability of the assist slots. This restraint also dictates 1X-size reticle feature dimensions and the employment of high contrast imaging resists. A new approach entails significantly oversizing the desired feature and introducing a phase-shifting region around the periphery. This type of structure affords substantial focus-exposure improvements and may either be fabricated in a single-level, self-aligned scheme or by a two-level exposure with conventional e-beam tools since the phase-shifting regions are on the order of 1 micrometers (reticle dimensions). Extensive modeling of this structure for isolated contact holes and spaces explores the myriad of trade- offs involved in an optimum design. Mask-fabrication tolerances, such as phase-shift uniformity, are also investigated. It is shown that the focus-exposure window enlarges as the overall structure dimensions increase. The degree of enhancement must therefore by weighed against packing density restrictions. Also, the structure suffers, to some degree, from the effect of side-lobes. However, for a given side-lobe intensity, this technique yields enhancements superior to the assist-slot approach. As is typical of phase-shifted systems, performance is improved as the partial coherence ((sigma) ) of the illuminating radiation is reduced. The decrease in throughput sometimes associated with a (sigma) reduction is, in this case, however, mitigated by the oversized aperture that produces twice the illuminating intensity as the corresponding non-phase-shifted feature. Simulated exposure-focus analysis conclude that a 0.45 (lambda) /NA contact hole may be printed with a 15% exposure and +/- .42 K2 unit focus tolerance assuming a +/- 5% CD control. A demonstration mask was patterned with a MEBES III generation reticle writer and exposure-focus latitude predictions for phase-shifting spaces are verified on an I-line, (sigma) equals0.5, 0.45 N.A. stepper.
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