Effect of lens aberration on oblique-illumination stepper system

Abstract

In this paper, detailed simulation and some experimental studies on stepper lens aberration effect in the case of oblique illumination source will be presented. The results are compared to that of conventional illumination source. Due to the unique feature of oblique illumination source imaging, i.e., imaging by using only zero and 1st diffraction order light, both stepper resolution limit and depth of focus (DOF) are extended. As a result, the effect of lens aberration in resist printing are also different from that of conventional illumination source. Unlike the conventional illumination source, the net effect of stepper lens aberration in resist printing depends not only on both the amount and type of the lens aberration, but also on the mask feature pattern. For example, in the case of nested lines, or periodic contact holes, the aerial image degradation as a result of lens aberration is less than that of conventional illumination source for spherical aberration, astigmatism, coma, field curvature, and chromatic aberration (in the case of excimer laser lithography). The differences in aerial image degradation between the two type of illumination sources (oblique and conventional) become less and less as the period of the nested line (with fixed linewidth) gets larger and larger. For other patterns, such as isolated lines and contacts, the effect of these lens aberrations in the oblique illumination source case is about the same as that of conventional illumination source. In the case of lens distortion, unlike the other types of lens aberration, the oblique illumination source does not show any improvement as compared to that of conventional illumination source. It does not show pattern dependent distortion either. In the experiment, an effect of a stepper lens aberration in resist printing for both conventional illumination and quadrapole illumination sources (mostly astigmatism) were measured. The results were in agreement with our simulation results. INTRODUCTIONIn a typical conventional projection stepper, the illumination source is designed to be circular with partial coherence values of 0.5-0.7. The resolution and DOF of mask pattern printed by such a stepper follows Rayleigh resolution and DOF criteria given bywhere k1 and k2 are the Rayleigh coefficients for minimum resolution and corresponding acceptable DOF, respectively. For typical i-line resist process, we have k1 0.7, and k2 -0.8, respectively. As the requirement of minimum feature size of IC pattern becomes 035 pm or smaller, based on Equation (1), higher NA lens is needed for i-line exposure wavelength in order to obtained the desired resolution. However, from Equation (2), the DOF will be reduced correspondingly. As a result, implementation of conventional i-line lithography in 64 Mbit DRAM production becomes very difficult without applying phase shift mask or any other lithographic enhancement technique. In the past few years, different lithographic enhancement techniques such as annular and quadrapole illumination sources were proposed and ...

<title>Fast algorithms for 3D high-NA lithography simulation</title>

“…For an ideal lens, the angular spectrum in the image space {A1} is the subset of the Fourier coefficients of the mask which satisfies eqn (1). The corresponding plane waves in the image space can be expressed as A1 • exp [_i27r(fx +f,y +fz)] (2) where f , A are the spatial frequencies of the plane wave, which are constrained by:…”

“…The imaging lens acts as a low-pass filter operating on the diffracted spatial frequency spectrum. If a spatial frequency component of the mask falls outside of the circle defined by = (4)2 (1) where NA is the numerical aperture of the lens, that frequency component is filtered out by the lens aperture.…”

“…Advanced modeling capabilities such as mask topography scattering and non-planar substrate are implemented by facilitating data transfer with a Maxwell's equations solver. The simulator is used extensively in research and process development and has produced many useful results [1] [2]. Figure 1 shows a simplified drawing of the projection lithography system.…”