Transport of intensity phase imaging in the presence of curl effects induced by strongly absorbing photomasks

Shanker, Aamod; Tian, Lei; Sczyrba, Martin; Connolly, Brid; Neureuther, Andrew R.; Waller, Laura

doi:10.1364/ao.53.0000j1

Cited by 25 publications

(12 citation statements)

References 37 publications

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“…Measuring phase edge effects from through-focus AIMS images a) For on-axis illumination, a stack of throughfocus intensity images is used to recover the phase at wafer plane. 8 Polarization dependence of topography induced phase is evident, in agreement with the boundary layer model of Fig 1c. b) AIMS tool configured for illumination with an off-axis monopole; the recovered phase is no longer directly indicative of mask diffraction since the pupil shift due to oblique incidence has a stronger phase signature than the thick mask edge phase.…”

Section: Introductionsupporting

confidence: 86%

“…The Transport of Intensity Equation (TIE) 12, 13 relates the intensity stack at wafer to the phase at focus; here we use a modified solver for the TIE developed for use with strongly absorbing photomasks. 8 This gives a first estimate of the wafer plane phase (after removing the fixed phase ramp across the field of view due to the illumination angle). Figure 6.…”

Section: Phase Retrieval Algorithmmentioning

confidence: 99%

“…1c), 4,5 or Zernike aberrations 6 in the imaging pupil. 1,7 We have shown 8,9 that edge effects can instead be visualized in experiments with an AIMS tool using phase imaging at the wafer plane (Fig. 2a).…”

Section: Introductionmentioning

confidence: 99%

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Characterizing the dependence of thick-mask edge effects on illumination angle using AIMS images

et al. 2015

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Mask topography contributes diffraction-induced phase near edges, affecting the through-focus intensity variation and hence the process window at the wafer. We analyze the impact of edge diffraction on projection printing directly with experiments on an aerial image measurement system (AIMS). We show here that topographic effects change with illumination angle and can be quantified using through-focus intensity measurements. Offaxis incidence influences not just defocus image behavior (as for normal incidence), but also the at-focus intensity at wafer. Moreover, with oblique illumination, mask diffraction varies for left-facing and right-facing sidewalls, the nature of the asymmetry being polarization dependent. The image degradation due the polarization parallel to the sidewall (TE) is seen to be stronger, owing to the interplay of mask topography and pupil filtering in the imaging system. This translates to a CD variation of 2% between the two polarizations, even at focus. A simple thin-mask boundary layer model that treats each sidewall independently is shown to be able to approximate mask topography induced diffraction for both polarizations with 5-10nm wide boundary layers.

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Section: Introductionsupporting

confidence: 86%

Section: Phase Retrieval Algorithmmentioning

confidence: 99%

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Characterizing the dependence of thick-mask edge effects on illumination angle using AIMS images

et al. 2015

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“…1) is solved in order to recover the phase at-focus from this dataset (Fig. 10b) using an iterative algorithm 17,18 that has the added advantage of being robust to reasonable amounts of partial coherence.…”

Section: Experimental Verification With Aims Toolmentioning

confidence: 99%

Absorber topography dependence of phase edge effects

Shanker

Sczyrba

Connolly³

et al. 2015

SPIE Proceedings

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Mask topography contributes to phase at the wafer plane, even for OMOG binary masks currently in use at the 22nm node in deep UV (193nm) lithography. Here, numerical experiments with rigorous FDTD simulation are used to study the impact of mask 3D effects on aerial imaging, by varying the height of the absorber stack and its sidewall angle. Using a thin mask boundary layer model to fit to rigorous simulations it is seen that increasing the absorber thickness, and hence the phase through the middle of a feature (bulk phase) monotonically changes the wafer-plane phase. Absorber height also influences best focus, revealed by an up/down shift in the Bossung plot (linewidth vs. defocus). Bossung plot tilt, however, responsible for process window variability at the wafer, is insensitive to changes in the absorber height (and hence also the bulk phase). It is seen to depend instead on EM edge diffraction from the thick mask edge (edge phase), but stays constant for variations in mask thickness within a 10% range. Both bulk phase and edge phase are also independent of sidewall angle fluctuation, which is seen to linearly affect the CD at the wafer, but does not alter wafer phase or the defocus process window. Notably, as mask topography varies, the effect of edge phase can be replicated by a thin mask model with 8nm wide boundary layers, irrespective of absorber height or sidewall angle. The conclusions are validated with measurements on phase shifting masks having different topographic parameters, confirming the strong dependence of phase variations at the wafer on bulk phase of the mask absorber.

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“…In some cases, partially coherent light is unavoidable. For example, in lithography applications [32], current trends towards source-mask optimization (SMO) [33,34] mean that non-traditional source shapes are the norm. Thus, incorporating partial coherence may be not only desirable, but necessary, for phase retrieval.…”

Section: Introductionmentioning

confidence: 99%

Partially coherent phase imaging with simultaneous source recovery

Zhong¹,

Tian²,

Dauwels³

et al. 2014

Biomed. Opt. Express

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We propose a new method for phase retrieval that uses partially coherent illumination created by any arbitrary source shape in Köhler geometry. Using a stack of defocused intensity images, we recover not only the phase and amplitude of the sample, but also an estimate of the unknown source shape, which describes the spatial coherence of the illumination. Our algorithm uses a Kalman filtering approach which is fast, accurate and robust to noise. The method is experimentally simple and flexible, so should find use in optical, electron, X-ray and other phase imaging systems which employ partially coherent light. We provide an experimental demonstration in an optical microscope with various condenser apertures.

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Transport of intensity phase imaging in the presence of curl effects induced by strongly absorbing photomasks

Cited by 25 publications

References 37 publications

Characterizing the dependence of thick-mask edge effects on illumination angle using AIMS images

Characterizing the dependence of thick-mask edge effects on illumination angle using AIMS images

Absorber topography dependence of phase edge effects

Partially coherent phase imaging with simultaneous source recovery

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