KEYWORDS: Optical proximity correction, Lithography, Signal processing, Image quality, Nanoimprint lithography, Photomasks, Calibration, System on a chip, Data modeling, Computer simulations
In this paper, we introduce a rigorous OPC technology that links the physical lithography simulation with the OPC. Firstly, the various aspects of the rigorous OPC, related to process flow, are discussed and the practical feasibility of the embedded rigorous verification is taken into account, which can make the rigorous treatment of the full-chip level possible without any additional manual efforts. We explain an embedded rigorous verification flow and the basic structure of its functionality. Finally, its practical application to real cases is discussed.
As IC design complexity keeps increasing, it is more and more difficult to ensure the pattern transfer after
optical proximity correction (OPC) due to the continuous reduction of layout dimensions and lithographic limitation by
k1 factor. To guarantee the imaging fidelity, resolution enhancement technologies (RET) such as off-axis illumination
(OAI), different types of phase shift masks and OPC technique have been developed. In case of model-based OPC, to
cross-confirm the contour image versus target layout, post-OPC verification solutions continuously keep developed -
contour generation method and matching it to target structure, method for filtering and sorting the patterns to eliminate
false errors and duplicate patterns. The way to detect only real errors by excluding false errors is the most important
thing for accurate and fast verification process - to save not only reviewing time and engineer resource, but also whole
wafer process time and so on. In general case of post-OPC verification for metal-contact/via coverage (CC) check,
verification solution outputs huge of errors due to borderless design, so it is too difficult to review and correct all points
of them. It should make OPC engineer to miss the real defect, and may it cause the delay time to market, at least.
In this paper, we studied method for increasing efficiency of post-OPC verification, especially for the case of
CC check. For metal layers, final CD after etch process shows various CD bias, which depends on distance with
neighbor patterns, so it is more reasonable that consider final metal shape to confirm the contact/via coverage. Through
the optimization of biasing rule for different pitches and shapes of metal lines, we could get more accurate and efficient
verification results and decrease the time for review to find real errors. In this paper, the suggestion in order to increase
efficiency of OPC verification process by using simple biasing rule to metal layout instead of etch model application is
presented.
In this paper, we discuss the accuracy of resist model calibration under various aspects. The study is done based on an
extensive OPC dataset including hundreds of CD values obtained with immersion lithography for the sub-30 nm
node. We address imaging aspects such as the role of Jones matrices, laser bandwidth and mask bias. Besides we focus
on the investigation on metrology effects arising from SEM charging and uncertainty between SEM image and feature
topography. For theses individual contributions we perform a series of resist model calibrations to determine their
importance in terms of relative RMSE (Root Mean Square Error) and it is found that for the sub-30 nm node they all are
not negligible for accurate resist model calibration.
This paper presents a novel mask corner rounding (MCR) modeling approach based on Synopsys' Integrated Mask and
Optics (IMO) modeling framework. The point spread functions of single, double, and elliptical Gaussians are applied to
the IMO mask kernels to simulate MCR effects. The simulation results on two dimensional patterns indicate that the
aerial image intensity variation is proportional to the MCR induced effective area variations for single type corners. The
relationship may be reversed when multiple types of corners exist, where the corners close to the maximum intensity
region have a greater influence than others. The CD variations due to MCR can be estimated by the effective area
variation ratio and the image slope around the threshold. The good fitting results on line-end patterns indicate that the
ΔCD is the quadratic function of the Gaussian standard deviations. OPC modeling on 28nm-node contacts shows that
MCR has significant impact on model fitting results and process window controls. By considering the real mask
geometry effects and allowing in-line calibration of model parameters, the IMO simulation framework significantly
improves the OPC model accuracy, and maintains the calibration speed at a good level.
As semiconductor manufacturing moves to 32nm and 22nm technology nodes with 193nm water immersion
lithography, the demand for more accurate OPC modeling is unprecedented to accommodate the diminishing
process margin. Among all the challenges, modeling the process of Chemically Amplified Resist (CAR) is a
difficult and critical one to overcome. The difficulty lies in the fact that it is an extremely complex physical and
chemical process. Although there are well-studied CAR process models, those are usually developed for TCAD
rigorous lithography simulators, making them unsuitable for OPC simulation tasks in view of their full-chip
capability at an acceptable turn-around time. In our recent endeavors, a simplified reaction-diffusion model capable
of full-chip simulation was investigated for simulating the Post-Exposure-Bake (PEB) step in a CAR process. This
model uses aerial image intensity and background base concentration as inputs along with a small number of
parameters to account for the diffusion and quenching of acid and base in the resist film. It is appropriate for OPC
models with regards to speed, accuracy and experimental tuning. Based on wafer measurement data, the parameters
can be regressed to optimize model prediction accuracy. This method has been tested to model numerous CAR
processes with wafer measurement data sets. Model residual of 1nm RMS and superior resist edge contour
predictions have been observed. Analysis has shown that the so-obtained resist models are separable from the effects
of optical system, i.e., the calibrated resist model with one illumination condition can be carried to a process with
different illumination conditions. It is shown that the simplified CAR system has great potential of being applicable
to full-chip OPC simulation.
In this paper, we present some important improvements on our process window aware OPC (PWA-OPC). First, a CDbased
process window checking is developed to find all pinching and bridging errors; Secondly, a rank ordering method
is constructed to do process window correction; Finally, PWA-OPC can be applied to selected areas with different
specifications for different feature types. In addition, the improved PWA-OPC recipe is constructed as sequence of
independent modules, so it is easy for users to modify its algorithm and build original IPs.
A variety of innovations including the reduction of actinic wavelength, an increase in lens NA, an
introduction of immersion process, and an aggressive OPC/RET technique have enabled device shrinkage
down to the current 45nm node. The immaturity of EUV and high index immersion, have made logic
manufacturers look at other ways of leveraging existing exposure technologies as they strive to develop
process technology for 32nm and below. For design rules for sub-nodes from 32nm to 22nm, the need to
define critical layers with double photolithography and etch process becomes increasingly evident.
Double patterning can come in a variety of forms or 'flavors'. For 32/28nm node, the patterning of 2D
features is so challenging that opposing line-ends can only be defined using an additional litho and etch
step to cut them. For 22nm node, even line/space gratings are below the theoretical k1=0.25 imaging limit.
Therefore pitch-doubling double patterning decomposition is absolutely required. Each double patterning
technology has its own set of challenges. Most of all, an existing design often cannot be shrunk blindly and
then successfully decomposed, so an additional set of restrictions is required to make layouts double
patterning compliant. To decompose a logic layout into two masks, polygons often need to be cut so that
they can be patterned using both masks. The electric performance of this cut circuitry may be highly
dependent on the quality of layout decomposition, the circuit characteristics and its sensitivity to
misalignment between the two patterning steps. We used representative logic layouts of metal level and
realistic models to demonstrate the issues involved and attempt to define formal rules to help enable lineend
splitting and pitch-doubling double patterning decomposition. This study used a variety of shrink
approaches to existing legacy layouts to evaluate double patterning compliance and a careful set-up of
parameters for the pitch splitting decomposition engine. The quality of the resultant imaging was tuned
using double patterning aware OPC and printability verification tools.
Due to shrinking design nodes and to some limitations of scanners, extreme off-axis illumination (OAI) required and
its use and implementation of assist features (AF) to solve depth of focus (DOF) problems for isolated features and
specific pitch regions is essential. But unfortunately, the strong periodic character of OAI illumination makes AF's print
more easily. Present OPC flows generate AFs before OPC, which is also causes some AF printing problems. At present,
mask manufacturers must downsize AF's below 30nm to solve this problem. This is challenging and increases mask cost.
We report on an AF-fixer tool which is able to check AF printability and correct weak points with minimal cost in
terms of DOF after OPC. We have devised an effective algorithm that removes printing AF's. It can not only search for
the best non-printing AF condition to meet the DOF spec, but also reports uncorrectable spots, which could be marked as
design errors. To limit correction times and to maximize DOF in full-chip correction, a process window (PW) model and
incremental OPC method are applied. This AF fixer, which suggests optimum AF in only weak point region, solves AF
printing problems economically and accurately.
Although the mask pattern created by fine ebeam writing is four times larger than the wafer pattern, the mask
proximity effect from ebeam scattering and etch is not negligible. This mask proximity effect causes mask-CD errors and consequently wafer-CD errors after the lithographic process. It is therefore necessary to include
the mask proximity effect in optical proximity correction (OPC). Without this, an OPC model can not predict
the entire lithography process correctly even using advanced optical and resist models. In order to compensate
for the mask proximity effect within OPC a special model is required along with changes to the OPC flow.
This article presents a method for producing such a model and OPC flow and shows the difference in results
when they are used.
Optical proximity correction (OPC) of contact-hole printing is challenging since its two dimensional shapes requires
through understanding of lithographic processes compared to one dimensional line and space pattering. Moreover,
recently, it is common to use "elongated contact holes" with large contact area, rather than simple circular ones, for small
electrical resistance. These elongated contact holes make it even more difficult to generate a good OPC model than the
circular ones because the elongated contact-hole patterning causes the asymmetric process effects. For example, impacts
of mask CD error, resist diffusion and resist development are different depending on the orientation of the elongated
contact holes. This paper presents how the OPC model for the elongated contact-hole can be improved as the mask CD
error compensation, accurate resist diffusion model and a new Variable Threshold Model (VTM) are applied for the
asymmetric process effects.
Many issues need to be overcome in creating a production-worthy sub-k1 (<0.25) process. The
repeating photo-etch sequential method for clear and dark mask type is susceptible to overlay
issues while accuracy of first pattern is critical for the space technology. Both technologies
require improved model accuracy and process margin. Because of this, even traditionally noncritical
regions of a layout may contain process margin-limiting defects for double patterning
technology. An integrated OPC-Verification-Selective OPC procedure is developed to improve
quality of results for non-critical regions while retaining fast TAT. The first step utilizes a fast
OPC method with reduced TAT. Next, a lithographic verification tool is used to perform a
thorough check of the OPC results, including process window analysis. This determines which
points limit process margin. Finally, advanced OPC methods are applied to reprocess the areas
limiting process margin. These advanced OPC techniques may include broader lithographic
analysis, field-based correction and process window consideration. Since advanced OPC
methods are only applied to part of the design, TAT is fast. TAT can be further improved by
treating critical regions differently. Critical regions will not be processed in the initial OPC or
intermediate verification steps, but will be corrected by the advanced OPC methods. This
methodology is called Incremental OPC as it applies the most appropriate OPC techniques to
each area of the design. As a result, process margin limiting defects, side-lobe printing and subresolution
assist feature printing can be eliminated prior to mask tape-out with minimal impact
to TAT. In this paper, Incremental OPC is compared to "all-or-nothing" OPC techniques which
must be applied across an entire pattern.
It is suggested that stray-light (SL, also called flare, scattered light) impact can be compensated by modifying standard
OPC method. Compared to traditional optical proximity effect caused by diffraction limit, stray light leads to extremely
long range (~ 100 micrometer ~ 10 millimeter) proximity effect. Appropriate approximation is introduced for stray-light
implemented OPC in such a large scale. This paper also addresses other practical problems in the stray-light OPC and
presents how to solve the problems.
Several criteria are applied to optimize the best illumination and bias condition for a layer. Normalized image log-slope
(NILS) and mask error enhancement factor (MEEF) are promising candidates to simply decide the optimized condition.
NILS represents imaging capability and MEEF represents the mask uniformity influence on wafer image. MEEF has
inversely relationship with NILS, but the optimized point of NILS does not exactly coincide with that of MEEF. Besides
NILS and MEEF, the depth of focus (DoF) is an important factor for defining the process margin. The process window
(PW) is expressed by DoF and exposure Latitude (EL). PW is general parameter used to determine the best lithographic
condition. Large EL can be obtained at the condition with good image performance. In order to include mask uniformity
effect in PW analysis, the common PW overlapping the final layout with positive and negative biased layouts is adopted.
Starting with the minimum NA, sigma and threshold, OPC is performed to satisfy the target layout using aerial image
model, and the final OPCed layout is obtained. The positive and negative biased layouts are generated from the final
OPCed layout. The bias limit is determined considering mask uniformity. The common PW obtained by overlapping the
final layout with positive and negative biased layouts is calculated. Then, NA, sigma and threshold are increased until
the maximum values are reached. The common PW at each NA, sigma and threshold value is obtained using the same
flow sequence. Comparing among calculated PWs, the NA, sigma and threshold of the maximum PW can be chosen as
the best illuminator and bias condition. In this paper, the optimized illumination and bias condition is determined using
PW for 60 nm memory device. The process flow is implemented by an OPC tool. By using the OPC tool for the
illuminator optimization, the actual layout and multiple monitoring points can be measured. In spite of a large number of
calculations, the fast calculation speed can be obtained by using the distributed process.
Boundary Layer Model (BLM) is applied to OPC for typical memory-device patterning processes for 3D mask
topographic effect. It is observed that this BLM successfully accounts for the 3D mask effect as reducing OPC model
error down to sub-50 nm node. BLM improves OPC-modeling accuracy depending on specific process conditions such
as mask type and pattern geometry. Potential limit of BLM, i.e., how accurately BLM could predict the 3D mask effect is
also investigated with respect to CD change: BLM also compared with rigorous simulation for various features and a
good match is obtained as small as below 0.5 nm. Some practical issue in OPC modeling such as determination of the
phase of boundary layer is addressed, which can be critical for prediction of defocus behavior.
We present simplified symmetric boundary layer model (BLM) for Optical Proximity Correction (OPC) in order to account for thick (or 3D or topographic) mask effect. In this approach, near-field mask image which is quite different from original mask pattern due to mask topography is approximated as the original pattern and boundary layer around it. In this work, the boundary layer is determined as such that residual critical dimension (CD) error between measured CD and modeled CD from the BLM is minimized for various types of features. In case of sub-50 nm memory patterning, this BLM shows sufficient accuracy that root mean square of the residual CD is as small as 4.3 nm. Also, OPC speed with BLM is reasonably fast as the OPC time with BLM increases as only around twice as the conventional OPC time without BLM, which is acceptable in practice.
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