KEYWORDS: Optical proximity correction, Data modeling, Critical dimension metrology, Optical calibration, Scanning electron microscopy, Hybrid optics, Metals, Calibration, Instrument modeling, OLE for process control
The model accuracy of optical proximity-effect correction (OPC) was investigated by two modeling methods for a 10-nm node process. The first method is to use contours of two-dimensional structures extracted from critical dimension-scanning electron microscope (CD-SEM) images combined with conventional CDs of one-dimensional structures. The accuracy of this hybrid OPC model was compared with that of a conventional OPC model, which was created with only CD data, in terms of root-mean-square (RMS) error for metal and contact layers of 10-nm node logic devices. Results showed improvement of model accuracy with the use of hybrid OPC modeling by 23% for contact layer and 18% for metal layer, respectively. The second method is to apply a correction technique for resist shrinkage caused by CD-SEM measurement to extracted contours for improving OPC model accuracy. The accuracy of OPC model with shrink correction was compared with that without shrink correction, and total RMS error was decreased by 12% by using the shrink correction technique. It can be concluded that the use of CD-SEM contours and the shrink correction of contours are effective to improve the accuracy of OPC model for the 10-nm node process.
KEYWORDS: Calibration, Data modeling, Cadmium sulfide, Optical proximity correction, Scanning electron microscopy, Statistical modeling, Finite element methods, Semiconducting wafers, Front end of line, Optical lithography
Continued improvements in SEM contour extraction capabilities have enabled calibrating more accurate OPC models for advanced technology nodes using a hybrid approach, combining CDs for 1D structures and full contour measurements for more complex 2D patterns. Previous work has addressed various components of contour modeling including alignment, edge detection, CD to contour consistency, and image parameter space coverage. This study covers weighting strategies for CDs compared to contours. Additionally the total number of structures in a sample plan can be reduced by incorporating contours for model calibration due to the increased number of evaluation points they provide.
Repeated measurements of the same structure at separate locations are used to extract SEM contours across several instances. The average measurements from these locations can then be used for OPC model calibration. Using 14nm process data, it is shown that including more contours in hybrid OPC model calibration leads to improved model verification. Within an appropriate range, higher weight on the contour patterns leads to improved model verification on measurement sites unseen by the calibration set. Calibrating a model with fewer contour structures, but at higher weight shows improvement over standard CD only model calibration.
KEYWORDS: Optical proximity correction, Data modeling, Scanning electron microscopy, Critical dimension metrology, Metals, Instrument modeling, Calibration, Metrology, Logic devices, OLE for process control
Hybrid OPC modeling is investigated using both CDs from 1D and simple 2D structures and contours extracted from complex 2D structures, which are obtained by a Critical Dimension-Scanning Electron Microscope (CD-SEM). Recent studies have addressed some of key issues needed for the implementation of contour extraction, including an edge detection algorithm consistent with conventional CD measurements, contour averaging and contour alignment. Firstly, pattern contours obtained from CD-SEM images were used to complement traditional site driven CD metrology for the calibration of OPC models for both metal and contact layers of 10 nm-node logic device, developed in Albany Nano-Tech. The accuracy of hybrid OPC model was compared with that of conventional OPC model, which was created with only CD data. Accuracy of the model, defined as total error root-mean-square (RMS), was improved by 23% with the use of hybrid OPC modeling for contact layer and 18% for metal layer, respectively. Pattern specific benefit of hybrid modeling was also examined. Resist shrink correction was applied to contours extracted from CD-SEM images in order to improve accuracy of the contours, and shrink corrected contours were used for OPC modeling. The accuracy of OPC model with shrink correction was compared with that without shrink correction, and total error RMS was decreased by 0.2nm (12%) with shrink correction technique. Variation of model accuracy among 8 modeling runs with different model calibration patterns was reduced by applying shrink correction. The shrink correction of contours can improve accuracy and stability of OPC model.
Techniques for identifying and mitigating effects of process variation on the electrical performance of integrated circuits
are described. These results are from multi-discipline, collaborative university-industry research and emphasize
anticipating sources of variation up-stream early in the circuit design phase. The lithography physics research includes
design and testing electronic monitors in silicon at 45 nm and
fast-CAD tools to identify systematic variations for entire
chip layouts. The device research includes the use of a spacer (sidewall transfer) gate fabrication process to suppress
random variability components. The Design-for-Manufacturing research includes double pattern decomposition in the
presence of bimodal CD behavior, process-aware reticle inspection, tool-aware dose trade-off between leakage and
speed, the extension of timing analysis methodology to capture across process-window effects and electrical processwindow
characterization.
This paper presents kernel convolution with pattern matching (KCPM), which is an updated version of fast-CAD
pattern matching for assessing lithography process variations. With KCPM, kernels that capture lateral feature
interaction between features due to process variations are convolved with a mask layout to calculate a match
factor, which indicates approximate change in intensity at the target location. The algorithm incorporates
a custom source, a mask with electromagnetic effects, and an arbitrary pupil function. For further accuracy
improvement, we introduce a source splitting technique. Though the evaluation speed is decreased, R2 correlation
of the match factor and change in intensity is increased. Results are shown with R2 correlation as high as 0.99 for
nearly coherent and annular illumination. Additionally, with a numerical aperture of 1.35, unbalanced quadrapole
illumination, 10mλ RMS random aberration in projection optics and complex mask with EMF effects included,
R2 correlation of more than 0.87 is achieved. This process is extremely fast (40μs per location) making it valuable
for a wide range of applications, most commonly hot spot detection and optimization.
In lithography for the 45nm node and beyond, phase errors introduced through electromagnetic field (EMF)
effects at photomask openings are significant sources of error in calculating on-wafer images. These edge effects
create distortion in both real and imaginary field transmission, which leads to a tilt in the process window, and
must be addressed in mask design to avoid loss of process latitude. This study presents a new formulation for
pattern matching, which allows EMF effects to be included via boundary layer modeling to facilitate extremely
fast assessment of EMF impact on imaging. Boundary layers are first used to model these edge effects, by
adding additional transmission features to a layout to represent the error transmissions caused by edges. Pattern
matching is then used to determine susceptibly to various pre-existing perturbations, in the presence of defocus.
This process can be extremely fast and hotspot detection can be run on an entire chip in hours, compared to
days for aerial imaging. Correlation between pattern matching and full aerial imaging can be as high as 0.97
for coherent imaging, and ≈ 0.75 for off-axis dipole illumination. This pattern matching framework is extremely
flexible and can be used for fast assessment of any series of effects which can be described as a path difference
in the pupil or as a transmission on the mask.
Simulation was used to explore boundary layer models for 1D and 2D patterns that would be appropriate for
fast CAD modeling of physical effects during design. FDTD simulation was used to compare rigorous thick mask
modeling to a thin mask approximation (TMA). When features are large, edges can be viewed as independent and
modeled as separate from one another, but for small mask features, edges experience cross-talk. For attenuating
phase-shift masks, interaction distances as large as 150nm were observed. Polarization effects are important
for accurate EMF models. Due to polarization effects, the edge perturbations in line ends become different
compared to a perpendicular edge. For a mask designed to be real, the 90o transmission created at edges
produces an asymmetry through focus, which is also polarization dependent. Thick mask fields are calculated
using TEMPEST and Panoramic Technologies software. Fields are then analyzed in the near field and on wafer
CDs to examine deviations from TMA.
This paper proposes a new highly sensitive scatterometry based Probe-Pattern Grating Focus Monitor. The high
sensitivity is achieved by placing transparent lines spaced at the strong focus spillover distance of around 0.6λ/NA from
the centerline of a 90 degree phase-shifted probe line that functions as an interferometer detector. The monitor translates
the focus error into the probe line trench depth, which can be measured by scatterometry techniques. Simulations of
optical imaging, resist development and Optical Digital Profilometry measurements are used to evaluate the expected
practical performance. A linear model is developed to estimate focus error based on the measured probe trench depth.
The results indicate that the ODP measurement from a single wafer focus setting can detect both the defocus direction
and the defocus distance to well under 0.1 Rayleigh unit of defocus.
An experimental technique for quantitatively characterizing edge effect contributions in transmission through thick
photomasks is described and evaluated through electromagnetic simulation. The technique consists of comparing the 0th
order transmission for various duty cycles to the expected experimental behavior from a thin mask model. The real
electric field component from the edges is proportional to the shift in the position of the minimum energy in the 0th order
field away from the expected thin mask location. The square root of the minimum 0th order diffraction energy
normalized to a clear mask gives the imaginary edge contribution. The results indicate that Alternating Phase Shifting
Masks (ALT-PSM) and Attenuating Phase Shifting Masks (ATT-PSM) technologies have significant edge effects on the
order of 0.1λ to 0.2λ per edge respectively, as well as polarization dependence. For periods of 2 wavelengths and larger
these edge contribution values are nearly independent of pitch. The existence of an imaginary (or quadrature) phase
component is shown to result in an additive linear variation of line edge shortening through focus. This tilt can be
interpreted as a focus shift of the normal parabolic behavior and is about 0.5 Rayleigh units (RU). This focus shift
depends to some extent on the surrounding layout as well as the feature itself.
Exploratory prototype DfM tools, methodologies and emerging physical process models are described. The examples
include new platforms for collaboration on process/device/circuits, visualization and quantification of manufacturing
effects at the mask layout level, and advances toward fast-CAD models for lithography, CMP, etch and photomasks. The
examples have evolved from research supported over the last several years by DARPA, SRC, Industry and the Sate of
California U.C. Discovery Program. DfM tools must enable complexity management with very fast first-cut accurate
models across process, device and circuit performance with new modes of collaboration. Collaborations can be promoted
by supporting simultaneous views in naturally intuitive parameters for each contributor. An important theme is to shift
the view point of the statistical variation in timing and power upstream from gate level CD distributions to a more
deterministic set of sources of variations in characterized processes. Many of these nonidealities of manufacturing can be
expressed at the mask plane in terms of lateral impact functions to capture effects not included in design rules. Pattern
Matching and Perturbation Formulations are shown to be well suited for quantifying these sources of variation.
Stray-light sources from pupil plane masks that may limit Terrestrial Planet Finder Coronagraph (TPF-C) performance are characterized1,2 and mitigation strategies are discussed to provide a guide for future development. Rigorous vector simulation with the Finite-Difference Time-Domain (FDTD) method is used to characterize waveguiding effects in narrow openings, sidewall interactions, manufacturing tool-marks, manufacturing roughness, mask tilt, and cross-wavelength performance of thick Silicon mask structures. These effects cause stray-light that is not accounted for in scalar thin-mask diffraction theory, the most important of which are sidewall interactions, waveguiding effects in narrow openings, and tilt. These results have been used to improve the scalar thin-mask theory used to simulate the TPF-C with the Integrated Telescope Model.3 Of particular interest are simulations of 100m thick vertical sidewall openings that model features typically found on Ripple masks4 fabricated by Reactive Ion Etching (RIE) processes.5 This paper contributes fundamental data for systematically modeling these effects in end-to-end system simulation.
Leakage straight through the mask material varies greatly with wavelength, especially in Silicon (an attractive mask material due to the precision manufacturing techniques developed by the IC industry). Coating Silicon with 200nm of Chrome effectively mitigates the leakage without causing additional scattering. Thick-mask diffraction differs from the predictions of scalar thin-mask theory because diffraction spreading is confined by the mask's sidewalls. This confinement can make a mask opening look electro-magnetically larger or smaller than designed, by up to 3λ per vertical sidewall on a 50μm thick mask yet this can be reduced an order of magnitude by undercutting the sidewalls 20°. These confinement effects are sensitive to mask tilt (if light reaches the sidewalls) which can lead to an imbalance in stray-light sources and an extra wavelength of effective opening change on the illuminated sidewall.
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