The physical process of mask manufacturing produces absorber geometry with significant deviations from the 90-deg corners, which are typically assumed in the mask design. The non-Manhattan mask geometry is an essential contributor to the aerial image and resulting patterning performance through focus. Current state-of-the-art models for corner rounding employ “chopping” a 90-deg mask corner, replacing the corner with a small 45-deg edge. A methodology is presented to approximate the impact of three-dimensional (3-D) EMF effects introduced by corners with rounded edges. The approach is integrated into a full-chip 3-D mask simulation methodology based on the domain decomposition method with edge to edge crosstalk correction.
The physical process of mask manufacturing produces absorber geometry with significantly less than 90 degree fidelity at corners. The non-Manhattan mask geometry is an essential contributor to the aerial image and resulting patterning performance through focus. Current state of the art models for corner rounding employ “chopping” a 90 degree mask corner, replacing the corner with a small 45 degree edge. In this paper, a methodology is presented to approximate the impact of 3D EMF effects introduced by corners with rounded edges. The approach is integrated into a full chip 3D mask simulation methodology based on the Domain Decomposition Method (DDM) with edge to edge crosstalk correction.
This study quantifies the impact of systematic mask errors on OPC model accuracy and proposes a methodology to reconcile the largest errors via calibration to the mask error signature in wafer data. First, we examine through simulation, the impact of uncertainties in the representation of photomask properties including CD bias, corner rounding, refractive index, thickness, and sidewall angle. The factors that are most critical to be accurately represented in the model are cataloged. CD bias values are based on state of the art mask manufacturing data while other variable values are speculated, highlighting the need for improved metrology and communication between mask and OPC model experts. It is shown that the wafer simulations are highly dependent upon the 1D/2D representation of the mask, in addition to the mask sidewall for 3D mask models. In addition, this paper demonstrates substantial accuracy improvements in the 3D mask model using physical perturbations of the input mask geometry when using Domain Decomposition Method (DDM) techniques. Results from four test cases demonstrate that small, direct modifications in the input mask stack slope and edge location can result in model calibration and verification accuracy benefit of up to 30%. We highlight the benefits of a more accurate description of the 3D EMF near field with crosstalk in model calibration and impact as a function of mask dimensions. The result is a useful technique to align DDM mask model accuracy with physical mask dimensions and scattering via model calibration.
This paper extends the state of the art by demonstrating performance improvements in the Domain
Decomposition Method (DDM) from a physical perturbation of the input mask geometry. Results from four
testcases demonstrate that small, direct modifications in the input mask stack slope and edge location can result in
model calibration and verification accuracy benefit of up to 30%. All final mask optimization results from this
approach are shown to be valid within measurement accuracy of the dimensions expected from manufacture. We
highlight the benefits of a more accurate description of the 3D EMF near field with crosstalk in model calibration
and impact as a function of mask dimensions. The result is a useful technique to align DDM mask model accuracy
with physical mask dimensions and scattering via model calibration.
The Domain Decomposition Method (DDM) for approximating the impact of 3DEMF effects was introduced nearly ten years ago as an approach to deliver good accuracy for rapid simulation of full-chip applications. This approximation, which treats mask edges as independent from one another, provided improved model accuracy over the traditional Kirchhoff thin mask model for the case of alternating aperture phase shift masks which featured severe mask topography. This aggressive PSM technology was not widely deployed in manufacturing, and with the advent of thinner absorbing layers, the impact of mask topography has been relatively well contained through the 32 nm technology node, where Kirchhoff mask models have proved effective. At 20 nm and below, however, the thin mask approximation leads to larger errors, and the DDM model is seen to be effective in providing a more accurate representation of the aerial image. The original DDM model assumes normal incidence, and a subsequent version incorporates signals from oblique angles. As mask dimensions become smaller, the assumption of non-interacting mask edges breaks down, and a further refinement of the model is required to account for edge to edge cross talk. In this study, we evaluate the progression of improvements in modeling mask 3DEMF effects by comparing to rigorous simulation results. It is shown that edge to edge interactions can be accurately accounted for in the modified DDM library. A methodology is presented for the generation of an accurate 3DEMF model library which can be used in full chip OPC correction.
The introduction of EUV lithography into the semiconductor fabrication process will enable a continuation
of Moore's law below the 22nm technology node. EUV lithography will, however, introduce new sources
of patterning distortions which must be accurately modeled and corrected with software. Flare caused by
scattered light in the projection optics result in pattern density-dependent imaging errors. The combination
of non-telecentric reflective optics with reflective reticles results in mask shadowing effects. Reticle
absorber materials are likely to have non-zero reflectivity due to a need to balance absorber stack height
with minimization of mask shadowing effects. Depending upon placement of adjacent fields on the wafer,
reflectivity along their border can result in inter-field imaging effects near the edge of neighboring
exposure fields. Finally, there exists the ever-present optical proximity effects caused by diffractionlimited
imaging and resist and etch process effects. To enable EUV lithography in production, it is
expected that OPC will be called-upon to compensate for most of these effects. With the anticipated small
imaging error budgets at sub-22nm nodes it is highly likely that only full model-based OPC solutions will
have the required accuracy. The authors will explore the current capabilities of model-based OPC software
to model and correct for each of the EUV imaging effects. Modeling, simulation, and correction
methodologies will be defined, and experimental results of a full model-based OPC flow for EUV
lithography will be presented.