Focus latitude enhancement of symmetrical phase mask design for deep submicron contact hole patterning

Abstract

The mechanism of focus latitude enhancement for contact/via hole printing is explained by approximating the axis intensity distribution of an image as a series of cosine functions to characterize the interference between each pair of diffraction beams. It is found that a phase-shifting mask ͑PSM͒ with symmetrical assist features improves the depth of focus ͑DOF͒ by introducing destructive interference to counterbalance the intensity fluctuation from constructive interference as defocus. A simple formula was derived to represent the capability of focus latitude enlargement. It shows that the extent of enhancement depends on the exposure wavelength and numerical aperture of a projection lens only. Increasing the degree of partial coherence degrades the focal range enlargement because a larger illumination angle elongates the destructive interference pattern in the optical-axis direction to weaken its ability for intensity compensation. On the other hand, the lack of constructive interference in dense hole imaging fails the mask pattern transfer, which limits the application of the phase-shifting method to pattern pitch greater than ͱ2/NA. A tiny amount of spherical aberration results in prominent asymmetrical defocus behavior because the wave deformation in the projection lens shifts the distribution of constructive and destructive interference patterns to opposite defocus directions. The printing characteristics of 0.17 m contact using an 18% transmission, rim-type attenuated phase-shifting mask are investigated to corroborate our analysis of defocus behavior. The dependence of depth of focus on pattern duty is stressed to elucidate the difference in mechanisms of focus latitude improvements for a sparse hole and periodic dense hole.

Customized illumination aperture filter design for through-pitch focus latitude enhancement of deep submicron contact hole printing

“…10 The prototype of customized illumination aperture filter (CIF) for contact hole patterning. vious work, 25 we have demonstrated a higher degree of illumination incoherence can alleviate this anomalous defocus behavior at the cost of DOF degradation. In the CIF design, we can control the degree of partial coherence through adjusting the quadruple width q to balance the performance between focus shift suppression and DOF enhancement.…”

“…6͔. In our previous work, 25 we have shown that the DOF enhancement of the phase-shifting method comes from the destructive interference between the negative and positive mask frequencies to counterbalance the axis intensity variation as defocus. Increasing the degree of partial coherence degrades the focal range enlargement because a larger illumination angle elongates the destructive interference pattern in optical-axis direction so that its ability for intensity compensation is weakened.…”

Optimization of the depth of focus based on the analysis of the diffraction orders in the pupil plane

“…A mandatory step to further optimise the photolithographic process is the understanding of the optical phenomena in the pupil plane. Several papers have shown [5][6][7] that the optimal depth of focus (DOF) is reached when the beams captured by the pupil are symmetrical to the optical axis. This means that the shape of the beams incident onto the mask impact the distribution of the diffraction orders in the pupil plane.…”

“…The shape and the position of these beams in the pupil plane have a great impact on the lithographic performance: if two beams are symmetrical to the optical axis, the DOF is at maximum [5,6] (Fig. 1b).…”