Contour-based OPC modeling has recently arisen as an alternative to the conventional CD-based method. In this work, an innovative flow is proposed to improve the quality of the final calibrated model by using SEM image contours. Layout pattern sampling technique should be introduced into this flow, which could not only ensure adequate coverage including IPS and pattern diversity, but also minimize the data collection effort. In this study, we have developed an automated high-precision contour extraction method to obtain good and reliable contours that were in good agreement with traditional CD-SEM measurements. The OPC model calibration was built by using the high-precision SEM contours, and we compared the contour-based method with conventional CD measurements. Finally, the model error RMS of the calibration and verification process could be fed back to the layout pattern sampling, which could benefit the sustainable improvement of the predictive ability of the model.
This study introduces a novel self-adaptive Pattern-to-Pattern (P2P) inspection mode for wafer defect inspection. Different from the traditional Die-to-Die (D2D), Cell-to-Cell (C2C), and Die-to-Database (D2DB) inspection modes, the newly proposed P2P inspection mode has the advantages of no restriction on the inspection region and low dependence on image quality. It works with both SEM images and optical images from different inspection equipment. Using the design layout information, the inspection images are aligned with the design and divided into basic component patterns according to the geometric features of the design pattern. These components are then analyzed through similar pattern comparison to enable the inspection of unique and complex patterns. This self-adaptive method eliminates the influence of the manufacturing process variations by comparing aligned similar image patterns, thereby preventing the reporting of defects highly dependent on the inspection algorithm settings. To facilitate further analysis, a database of the basic pattern components is created by collecting extensive images along with corresponding design layout information.
As the semiconductor process technology steps into a more advanced node, design and process induced systematic defects become increasingly significant yield limiters. Focus Exposure Matrix (FEM) method is crucial for early detection of these defects. However, analysis of a typical FEM wafer which contains half million of defects requires extensive time and efforts. In order to improve FEM wafers review efficiency, we introduce a smart review point selection strategy based on different layout pattern grouping modes. This review point selection strategy enable engineers to sample the more effective defects for SEM review.
Traditional scanner matching methods have been based in 1D proximity matching targets
and the use of wafer-based CD metrology to characterize both the initial mismatch as
well as the sensitivity of CDs to scanner tuning knobs.
One such method is implemented in ASML Pattern Matcher, which performs a linear
optimization based on user provided CD sensitivities and pre-match data. The user
provided data usually comes from wafer exposures done at multiple scanner illumination
conditions measured with CD-SEM. In the near future ASML plans to provide the
capability to support YieldStar CD data for Pattern Matcher which will collect CD data
with higher precision and much faster turn-around-time that CD-SEM.
Pattern Matcher has been used successfully in multiple occasions. Results for one such
occasion are shown in Figure 1 which presents the through pitch mismatch behavior of
one ASML XT:1400F with respect to an ASML XT:1400E for a 32nm contact layer.
Proximity matching is a common activity in the wafer fabs1,2,3 for purposes such as
process transfer, capacity expansion, improved scanner yield and fab productivity. The
requirements on matching accuracy also become more and more stringent as CD error
budget shrinks with the feature size as technology advances. Various studies have been
carried out, using scanner knobs including NA, inner sigma, outer sigma, stage tilt,
ellipticity, and dose. In this paper, we present matching results for critical features of a
logic device, between an ASML XT:19x0i scanner and an XT:1700i (reference),
demonstrating the advantage of freeform illuminator pupil as part of the adjustable
knobs to provide additional flexibility. We also present the investigation of a novel
method using lens manipulators for proximity matching, effectively injecting scalar
wavefront to an XT:19x0i to mimic the behavior of the XT:1700i lens.
Scanner mismatch has become one of the critical issues in high volume memory production. There are several
components that contribute to the scanner CD mismatch. One of the major components is illumination pupil difference
between scanners. Because of acceleration of dimensional shrinking in memory devices, the CD mismatch became more
critical in electrical performance and process window.
In this work, we demonstrated computational lithography model based scanner matching for sub 3x nm memory devices.
We used ASML XT:1900Gi as a reference scanner and ASML NXT:1950i as the to-be-matched scanner. Wafer
metrology data and scanner specific parameters are used to build a computational model, and determine the optimal
settings by model simulation to minimize the CD difference between scanners. Nano Geometry Research (NGR) was
used as a wafer CD metrology tool for both model calibration and matching result verification. The extracted pupil
parameters from measured source map from both before and after matching are inspected and analyzed. Simulated and
measured process window changes by applying the matching sub-recipe are also evaluated.
Scanner matching based on wafer data has proven to be successful in the past years, but its adoption into production has
been hampered by the significant time and cost overhead involved in obtaining large amounts of statistically precise
wafer CD data. In this work, we explore the possibility of optical model based scanner matching that maximizes the use
of scanner metrology and design data and minimizes the reliance on wafer CD metrology.
A case study was conducted to match an ASML ArF immersion scanner to an ArF dry scanner for a 6Xnm technology
node. We used the traditional, resist model based matching method calibrated with extensive wafer CD measurements
and derived a baseline scanner manipulator adjustment recipe. We then compared this baseline scanner-matching recipe
to two other recipes that were obtained from the new, optical model based matching method. In the following sections,
we describe the implementation of both methods, provide their predicted and actual improvements after matching, and
compare the ratio of performance to the workload of the methods. The paper concludes with a set of recommendations
on the relative merits of each method for a variety of use cases.
At 65nm technology node and below, with the ever-smaller process window, it is no longer sufficient to apply traditional
model-based verification at only the nominal condition. Full-chip, full process-window verification has started to
integrate into the OPC flow at the 65nm production as a way of preventing potentially weak post-OPC designs from
reaching the mask making step. Through process-window analysis can be done by way of simulating wafer images at
each of the corresponding focus and exposure dose conditions throughout the process window using an accurate and
predictive FEM model. Alternatively, due to the strong correlation between the post-OPC design sensitivity to dose
variation and aerial image (AI) quality, the study of through-dose behavior of the post-OPC design can also be carried
out by carefully analyzing the AI. These types of analysis can be performed at multiple defocus conditions to assess the
robustness of the post-OPC designs with respect to focus and dose variations. In this paper, we study the AI based
approach for post-OPC verification in detail.
For metal layer, the primary metrics for verification are bridging, necking, and via coverage. In this paper we are mainly
interested in studying bridging and necking. The minimum AI value in the open space gives an indication of its
susceptibility to bridging in an over-dosed situation. Lower minimum intensity indicates less risk of bridging.
Conversely, the maximum AI between the metal lines provides indication of potential necking issues in an under-dosed
situation.
At times, however, in a complex 2D pattern area, the location as to where the AI reaches either maximum or minimum is
not obvious. This requires a full-chip, dense image-based approach to fully explore the AI profile of the entire space of
the design. We have developed such an algorithm to find the AI maximums and minimums that will bear true relevance
to the bridging and necking analysis. In this paper, we apply the full-chip image-based analysis to 65nm metal layers.
We demonstrate the capturing of potential bridging or necking issues as identified by the AI analysis. Finally, we show
the performance of the full-chip image-based verification.
Lithography simulation is an integral part of semiconductor manufacturing. It is not only required in lithography process development, but also in RET design, RET verification, and process latitude analysis, from library cells to full-chip tape out. Two RET design checking flows are examined and compared. In the first flow, an image contour is simulated from post-OPC, GDSII data at best focus and exposure conditions. RET design defects are identified by comparing the calculated contours with the pre-OPC design data. To check lithography manufacturability across the typical IC process window, the second RET verification flow simulates image contours at multiple focus and exposure conditions. These RET design checking flows are implemented on new platform that combines a hardware accelerated computational engine with a new analysis method to numerically evaluate the lithographic printing and mask manufacturing challenges for a given design layout. The algorithm approach in this new system is based on image processing which is fundamentally different from conventional edge-based analysis. Specific examples of a mask aware RET verification flow leveraging this new platform and method will be provided with speed and accuracy benchmarks. Through the high speed computation of lithographic images from full chip data, many opportunities for novel and cost effective post layout lithography verification options become available. By combining the new platform with analysis steps relevant in leading edge photomask manufacturing, it may become possible to reduce the risks inherent in advanced technology tape outs while improving layout to mask fabrication cycle time and cost.
With expected implementation of low k1 lithography on 193nm scanners for 65nm node wafer production, high resolution defect inspection will be needed to insure reticle quality and reticle manufacture process monitoring. Reticle cost and reticle defectivity are both increasing with each shrink to the next node. Simultaneously, system on chip (SoC) designs are increasing in which a large area of the exposure field typically contains dummy patterns and other features which are not electrically active. Knowing which defects will electrically impact device yield and performance can improve reticle manufacturing yield and cycle time -- resulting in lower reticle costs. This investigation examines the feasibility of using additional design data layers for die-to-database reticle inspection to determine in real time the relevance of a reticle defect by its location in the device (Smart InspectionTM). The impact to data preparation and inspection throughput is evaluated. The current prototype algorithm is built on the XPA and XPE die-to-database algorithms for chrome-on-glass and EPSM reticles, respectively. The algorithms implement variable sensitivity based on the additional design data regions. During defect review the defects are intelligently binned into the different predetermined design regions. Tests show the new Smart Inspection algorithm provides the capability of using higher than normal sensitivity in critical regions while reducing sensitivity in less critical regions to filter total defect counts and allow for the review of just defects that matter.
Performance characterization of a variable sensitivity Smart Inspection algorithm is discussed in addition to the filtering of the total defect count during review to show the defects that matter to device performance. Using seven critical layer production reticles from a system on chip device we examine the applications of Smart Inspection by layer including active, poly, contact, metal and via layers. Data volume for additional data layers show little impact to inspection data prep time. The total area of the reticle where defects do not matter is as high as 70% on some layers. Review capabilities will be examined for various applications such as reviewing defects in the various regions such as SRAM, dummy pattern, and redundant contact/via specified regions. Lastly, the economics of Smart Inspection will be modeled using the collected knowledge of the applications from the production reticle characterized in this investigation.
Contacts and VIAs are features whose integrity are very susceptible to reticle CD defects or in general, to defects that produce a change of total energy (flux) projected through the reticle. As lithography is extended beyond the 130nm node, the problem becomes more critical. Detecting and analyzing photomask critical dimension (CD) errors and semitransparent defects is vital for qualifying reticles to enable high IC wafer yield for the 90nm node. The current state of the art inspection methods are unable to meet the industry requirements for contact and via features. Using the TeraStarTM pattern inspection system's image computer platform, a new algorithm, TeraFluxTM, has been implemented and tested for the inspection of small 'closed' features. The algorithm compares the transmitted energy flux difference between a test contact (or a group of contacts) and a reference image for small closed features, such as, contacts, trenches, and cells on chrome and half-tone reticles. The algorithm is applicable to both clear and dark field reticles. Sensitivity characterization tests show that the new algorithm provides CD error detection to 6% energy flux variation with low false defect counts. We performed experiments to correlate the sensitivity performance of the new algorithm with wafer printability results. The results will be presented together with results of inspections results of programmed defect plates and production reticles.
For alternating aperture phase shift masks (AAPSM) and 193 nm (ArF) lithography, we have simulated defect printability using inspection images and software-based modeling. Masks were fabricated by DuPont Photomasks with programmed defects of known size, phase, and location. Three phase layers were used to generate defect angles 60, 120 and 180 degrees. Simulated wafer prints were performed using Numerical Technologies’ Virtual Stepper System, which takes inspection images as input and models the lithographic process. With inspection images from KLA-Tencor’s SLF27 system, our critical-dimension measurements show good agreement with those from wafers printed on an ASML PAS 5500/900 scanner.
With growing implementation of low k1 lithography on DUV scanners for wafer production, detecting and analyzing photomask critical dimension (CD) errors and semitransparent defects is vital for qualifying photomasks to enable high IC wafer yields for 130nm and 100nm nodes. Using the TeraStar pattern inspection system's image computer platform, a new die-to-database algorithm, TeraFlux, has been implemented and tested for the inspection of small "closed" features. The algorithm is run in die-to-database mode comparing the energy flux difference between reticle and the database reference for small closed features, such as, contacts, trenches, and cells on chrome and half-tone reticles. The algorithm is applicable to both clear and dark field reticles. Tests show the new algorithm provides CD error detection to 6% energy flux variation with low false defect counts.
We have characterized the sensitivity and false defect performance of the die-to-database energy flux algorithm with production masks and programmed defect test masks. A sampling of inspection results will be presented. Wafer printability results using the programmed defects on a programmed defect test reticle will be presented and compared to the inspection defect sensitivity results.
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