| The continuing trend toward smaller dimensions in integrated circuit fabrication has led to the development of diffraction-limited lithographic projection systems capable of resolving features on the order of the source wavelength over field sizes of several square centimeters. The decrease in the size of the smallest resolvable features has been accompanied by a continuous reduction in the available depth of focus in the aerial image, to less than one micron in the most advanced systems. Since in many lithographic applications the height of the substrate topography is on the same order, depth of focus has become the most critical issue challenging the future of optical lithography.; Several techniques to overcome this limitation have been suggested prior to this work. We have investigated one of these, pupil filtering, as a spatial frequency domain approach, and devised a new technique, two-level masks, as a space domain approach to extending the depth of focus. In studying pupil filtering, we have examined the trade-off between the possible improvements in depth of focus and the necessary reduction in radial energy concentration in the three-dimensional point spread function. Based on this trade-off, we have devised an optimization procedure for radially symmetric filter functions. To verify our predictions experimentally we have fabricated filters corresponding to the calculated pupil functions and measured the resulting three-dimensional point spread functions. We have also applied these to simple test patterns and demonstrated improved depth of focus.; To examine the potential benefits of two-level mask structures we have devised a fast, approximate simulation algorithm based on Kirchhoff boundary conditions and checked its predictions using more exact, iterative calculations. Our simulations indicated improvements in depth of focus for a variety of pattern types. To demonstrate this technique in practice, we have fabricated a two-level mask containing test patterns applicable to the coil level in magnetic thin film head lithography. Through-focus series of resist exposures were performed and have shown an approximately two-fold improvement in depth of focus over the results obtainable from conventional masks. |
