A methodology to determine the optimum measurement condition of extreme ultraviolet (EUV) resist patterns in a critical dimension scanning electron microscope has been established. Along with many parameters that need to be optimized simultaneously, there are conflicting requirements of small resist shrinkage and high measurement precision. To overcome these difficulties, we have developed a methodology for ArF resist patterns from shrinkages and precisions predicted by the Taguchi method. In this study, we examined the extendibility of the methodology to sub-20 nm EUV resist patterns. The predicted shrinkage by the Taguchi method for an 18 nm EUV resist pattern showed a large prediction error due to its different dependence on acceleration voltage from ArF, so we used the shrinkage curve to predict shrinkage instead of the Taguchi method, as shrinkage depends only on irradiated electron dose. In contrast, precision can be predicted well by the Taguchi method as with ArF. We propose a methodology that consists of separate prediction procedures for shrinkage and precision using the shrinkage curve and Taguchi method, respectively. The proposed method was applied to an 18-nm EUV resist pattern. The optimum measurement condition with shrinkage of 1.5 nm and precision of 0.12 nm was determined.
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.
A novel methodology was established for determining critical dimension scanning electron microscope (CD-SEM) optimum measurement condition of sub-20 nm resist patterns for 0.33NA EUV lithography yielding both small shrinkage and high precision. To investigate dependency of resist shrinkage on pattern size and electron beam irradiation condition, shrinkage of 18, 32, and 45 nm EUV resist patterns was measured over a wide range of beam conditions. A shrinkage trend similar to that of ArF resist patterns was observed for 32 and 45 nm, but 18 nm pattern showed a different dependence on acceleration voltage. Conventional methodology developed for ArF resist pattern to predict shrinkage and precision using the Taguchi method was applied to EUV resist pattern to examine the extendibility of the method. Predicted shrinkage by Taguchi method for 32 and 45 nm patterns agreed with measurements. However, the prediction error increases considerably as the pattern size decreases from 32 to 18 nm because there is a significant interaction between acceleration voltage and irradiated electron dose in L18 array used in the Taguchi method. Thus, we proposed a new method that consists of separated prediction procedures of shrinkage and precision using both a shrinkage curve and the Taguchi method, respectively. The new method was applied to 18 nm EUV resist pattern, and the optimum measurement condition with shrinkage of 1.5 nm and precision of 0.12 nm was determined. Our new method is a versatile technique which is applicable not only to fine EUV resist pattern but also to ArF resist pattern.
We have proposed a new method for correcting electron beam (EB)-induced photoresist shrinkage in two-dimensional pattern contours extracted from a scanning electron microscope image. This method restores the original shrinkage-free contour from the experimentally determined “shrunk contour”, based on a shrinkage model which takes into account of the elastic nature of the shrinkage phenomena caused by the photoresist-volume reduction. Verification of this shrinkage model was demonstrated by using ArF resist patterns as follows. First, the model was calibrated with the shrinkage data of several line patters with different linewidth prior to the contour correction. Next, the amount of shrinkage of elbow patterns was measured by comparing its contours obtained with small and sufficiently large EB dosages. It was found that the shrinkage of the inner edge of the elbow corner was smaller than that of the outer edge, which can be interpreted as a result of the elastic deformation. Finally, validity of shrinkage correction was examined. The model calculation correctly reproduced the observed shrinkage including its dependence on the location in the pattern. The restored contour showed a good consistency with the experimental results and the total root-mean-square error of the shrinkage correction was 0.5 nm. This result confirmed that our shrinkage model adequately describes the shrinkage of two dimensional patterns. Consequently, proposed shrinkage correction method is expected to improve the accuracy of contour measurements by a critical dimension-scanning electron microscope.
Evaluation of resist shrinkage and precision by critical dimension scanning electron microscope (CD-SEM) for EUV resist patterns at around 20 nm exposed by 0.33 NA EUV tool was conducted. To investigate interaction between EUV resist and electron beam, an accurate and fast measurement method of resist shrinkage was established. Our method can avoid saturation of shrinkage at large dose conditions which was a demerit in conventional method. By applying the new method, pattern size dependence of shrinkage was measured with various line and space (L/S) patterns down to 20 nm. The result shows that resist shrinkage of fine L/S EUV resist pattern largely depends on line width rather than space width. A well-known trade-off relationship between shrinkage and precision was observed for EUV resist pattern as well as ArF resist pattern. Shrinkage of 1.6 nm and precision of 0.13 nm for 18 nm EUV resist pattern were obtained at a typical CD-SEM condition. We also measured shrinkage and precision for a dense L/S pattern at various exposure focus and dose conditions using a FEM wafer to examine the impact of process variability. To investigate the influence of EUV shadowing effect, we measured them for both horizontal and vertical patterns at different slit locations in exposure field. No systematic change of shrinkage and precision was observed through exposure focus and dose in the process window across slit location for both horizontal and vertical L/S patterns.
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.
A dose-focus monitoring technique using a critical-dimension scanning electron microscope (CD-SEM) is studied for applications on product wafers. Our technique uses two target structures: one is a dense grating structure for dose determination, and the other is a relatively isolated line grating for focus determination. These targets are less than 6 μm, and they can be inserted across a product chip to monitor dose and focus variation in a chip. Monitoring precision is estimated to be on the order of 1% for dose and 10 nm for focus, and the technique can be applied to dose and focus monitoring on product wafers. The developed technique is used to analyze spatial correlation in dose and focus over a wide range of distances, using a mask with a multitude of these targets. The variation (3σ ) of dose and focus difference between two monitor targets is examined for various separation distances, and the variation of focus difference increases from 10 to 25 nm as the separation distance increases from ∼20 μm to ∼10 mm . The variation of 10 nm observed at the shortest distance reflects focus monitoring precision, and focus variation sources such as wafer thickness variation come into play at longer distances.
ArF lithography is still the main technology in the most advanced processes of semiconductor fabrication. Being able to
reliably measure and characterize these lithographic processes in-depth is becoming more and more critical. Critical
Dimension-Scanning Electron Microscope (CD-SEM) continues to be the work horse tool for both in-line critical
dimension (CD) metrology and characterization of ArF photoresist pattern. CD shrink of ArF photoresist has been one of
the major challenges for CD-SEM metrology, and it becomes more difficult to measure shrinkage accurately for smaller
feature size than ~50nm. The authors have developed a new measurement technique of photoresist shrinkage which
measures CD difference between shrunk and non-shrunk sites after etching.
There are many imaging and image processing parameters in CD-SEM which need to be optimized to obtain small
shrinkage and good precision. There is a trade-off relationship between shrinkage and precision, and a comprehensive
and systematic methodology is required for optimization of parameters. The authors have developed an optimization
method that uses Taguchi method, where only 18 experiments are required. We can predict shrinkage, precision and
relative CD offset for any combination of measurement parameter settings used in the 18 experiments by Taguchi
method, and these predicted data can be used for optimization. A new concept of secondary reference metrology is also
introduced in this paper to reduce the number of measurement by a reference metrology tool.
A dose-focus monitoring technique using critical dimension scanning electron microscope (CD-SEM) is studied for onproduct
applications. Our technique uses two target structures; one is a dense grating structure with iso-focal pitch for
dose determination, and the other is a relatively isolated line grating with no assists for focus determination. The small
sizes of these targets enable us to monitor dose and focus variations across the chip on a product wafer. The model
which describes how the top and bottom CD depend on dose and focus deviations is the same as that for scatterometry
dose-focus metrology, and monitoring precision is estimated to be the order of 1% for dose and 10~15nm for focus. The
method has strong potential to apply to dose and focus monitoring of product wafers.
By using a mask with a multitude of these targets, it is possible to study dose and focus variations across the wafer in
great detail. The focus variation of pairs of such targets is measured for various separations between the two targets. As
the separation distance increases from ~100μm to ~10mm, the focus variation increases from 10nm to 25nm. We think
that the true focus variation between targets becomes near zero at the small separation distance, while the focus variation
increases as separation distance increases because more variation sources such as wafer thickness variation are included
at larger separation distances. Our small CD-SEM targets allow us to explore this kind of local spatial variation analysis.
We have developed a new local overlay measurement technique on actual device patterns using a critical dimension scanning electron microscope (CD-SEM), which can be applied to two-dimensional (2D) device structures such as a static random access memory contact hole array. CD-SEM overlay measurement can provide additional local overlay information at the site of device patterns, complementary to the optical overlay. The methodology includes the use of pattern symmetry to cancel out many process effects and reduce measurement uncertainty. CD-SEM overlay metrology was compared with conventional optical overlay metrology in terms of measurement uncertainty and overlay model analysis, and very good correlation was confirmed. The developed methodology was applied to local overlay measurement of double patterning contact hole layers of leading edge devices. The local overlay distribution was obtained across the device area, and spatial correlation of the overlay error vectors was examined over a large range of distances. The applications of CD-SEM overlay metrology were explored, and methodologies were introduced to examine both the overlay of double patterning contacts at the edge of an array and lithographic process-induced overlay shift of contacts. Finally, a hybrid optical CD-SEM overlay metrology was introduced in order to capture a high order, device weighted overlay response.
This work explores the applications of CD-SEM overlay metrology for double patterned one-dimensional (1D) pitch
split features as well as double patterned ensembles of two-dimensional (2D) complex shapes. Overlay model analysis
of both optical overlay and CD-SEM is compared and found to give nearly equivalent results. Spatial correlation of the
overlay vectors is examined over a large range of spatial distances. The smallest spatial distances are shown to have the
highest degree of correlation. Correlation studies of local overlay in a globally uniform environment, suggest that the
smallest sampling of overlay vectors need to be ~10-15μm, within the spatial sampling of this experiment. The smallest
spatial distances are also found to have to tightest mean distributions. The distribution width of the CD-SEM overlay is
found to scale linearly with log of the spatial distances over 4-5 orders of magnitude of spatial length.
Methodologies are introduced to examine both the overlay of double pattern contacts at the edge of an array and
lithographic process-induced overlay shift of contacts. Finally, a hybrid optical- CD-SEM overlay metrology is introduced in order to capture a high order, device weighted overlay response.
We have developed a new local overlay measurement technique on actual device patterns using critical dimension
scanning electron microscope (CD-SEM), which can be applied to 2D device structures such as an SRAM contact hole
array or more complex shapes. CD-SEM overlay measurement can provide additional local overlay information at the
site of device patterns, complementary to the conventional optical overlay data. The methodology includes the use of
symmetrically arranged patterns to cancel out many process effects and reduce measurement uncertainty. The developed
methodology was applied to local overlay measurement of double patterning contact hole layers of leading edge devices.
Local overlay distribution was successfully captured on device structures on different length scale, and the result shows
the possibility of assessing process induced shift on device structures and collecting denser sampling for better intra-chip
overlay control.
The measurement uncertainty of CD-SEM overlay metrology was assessed by comparing with conventional optical
overlay metrology for 1D and 2D structures. Very good correlation was confirmed between SEM and optical overlay
metrology with net residual error of ~1.1nm. Measurement variation associated with pattern roughness was analyzed for
1D structure, and identified as one of major variation sources for CD-SEM overlay metrology.
We have developed a resist-shade mask (R-mask) technology applicable for small-volume production. The R-mask uses a novel resist as a shading material instead of chromium (Cr), and it exhibits sufficient durability against KrF exposure for ASIC and pilot line applications. Because the R-mask does not require a Cr etching process, it can reduce mask costs and improve critical dimension (CD) uniformity. A defect inspection technique for R-masks has also been investigated, and no defects were observed on a wafer for several R-masks used for device fabrication. The part of the R-mask making contact in exposure tools was carefully designed to not retain resist material so as to avoid particle contamination. We applied several R-masks to form wiring layers for 0.25-um and 0.18-um logic devices and confirmed that there were no differences in process margin and product yield between the R-masks and conventional Cr masks.
We have also developed the partial R-mask, which consists of both conventional Cr mask and R-mask areas. The partial R-mask is very effective for customizing semiconductor chips. The R-mask area is applied only to customized circuit areas or certain wiring patterns to adjust circuit characteristics, whereas the common circuit area is delineated by the Cr pattern. The R-mask can be also used to customize attenuated phase-shifting masks, and to make unnecessary hole patterns opaque in prepared hole arrays.
The R-mask is a very promising technology for reducing mask costs and improving the turn-around time (TAT) of masks, because of its simple manufacturing process and reworkable capability.
We propose a new double exposure technique to obtain a balanced intensity profile through focus using an alternating phase-shifting mask (alt-PSM) with a reversed phase. To cancel the intensity imbalance caused by the mask topography and phase error, an additional alt-PSM which has a reversed phase is prepared and exposed at the same position on a wafer. In practical application, two alt-PSMs with reversed phase relative to each other are placed along the scan direction (y-direction) in a 4X-reticle. The imbalanced images are added in a complementary manner by repeating exposure at a half dose and step at a half pitch along the y-direction. The throughput loss can be minimized by using a chip layout with two alt-PSMs in one reticle. The impact of position error between two exposures on lithography performance is discussed, and a 20-nm position error is shown to be tolerable for 80-nm L/S patterns. Both theoretical discussions and experimental data show that even a no Cr-undercut design and a 10° phase error are acceptable. Also, this double exposure technique can lower the risk of defect printing. Such large tolerance regarding the topographical design, phase error, and phase defects is the key to the application of alt-PSM technology in low k1 lithography beyond the 65-nm node.
Mask quality issues in pushing lithography to features below 0.5(lambda) /NA are identified and quantified through simulation of mask interactions and images. Guidelines summarize the results from detailed studies of aberrations, phase-shift mask image imbalance, 3D phase defects and EUV buried defects. Programmed-probe based aberration targets are extended to distinguish both even and odd lens aberrations and their mask tolerance requirements are assessed. Complex diffraction coefficients and results for cross-talk simulation are used to set guidelines for phase-shifting mask design. An antireflection coating (50 nm MoO3) is shown to reduce cross-talk between trenches. Type, location and size data are given for 3D phase-defects and the end regions of lines are shown to be more vulnerable to CD variation. Results for buried 3D Gaussian defects in EUV multilayers show a worst isolated defect size of half of the resolution and that 2nm high defects of any size can be tolerated.
Programmed defects about 0.4 ?/NA in size are introduced in strong phase-shifting masks to produce exposure-sensitive printable artifacts for measuring lens aberrations. The programmed defects add interferrometric-like reference electric fields that coherently interact with the side-lobes of aberration sensitive pattern layouts to produce artifacts. The artifacts are separate but adjacent printed defects. The patterns are suitable for rapid reading by automatic wafer inspection equipment and directly indicate the levels of specific Zemike aberrations. High sensitivity to 0.01 X and good orthogonality with 12% confounding are possible for coma and trifoil. Results for even aberrations such as focus, astigmatism and spherical 3rd order are poor for 180° phase defect-probes but warrant further investigation with 90° phase.
We have investigated the effects of the topography of the phase-shifting mask on the aerial image characteristics in DUV lithography. The calculation of near fields is carried out through simulation of the mask with TEMPEST and linking the resultant near fields to EM-Aerial for imaging. It is shown that the Fourier spectrum for an alternating phase-shifting mask can be decomposed into Fourier spectra for single openings. The amplitude and phase of the diffraction orders for the single opening are utilized for the systematic analysis of the shifter edge topography. The analysis framework developed in this paper clearly identifies the effects of the wall of the phase shifter, the residual transmittance through the chromium area, and the cross-talk between adjacent features. This analysis framework also allows these effects be merged in design. The near field profile in the vicinity of the shifter wall is also investigated for different feature sizes, and the optimum design for different feature sizes is discussed. The effect of the wall angle profile is shown to be acceptable.
We investigated the critical dimension (CD) accuracy of 140- nm gate patterns fabricated by using the KrF phase-edge phase shifting mask (PSM), especially the dependence of the CD variation on the pattern pitch conditions, phase assignment conditions, and process conditions such as defocus and misalignment. We found that the phase assignment conditions for the neighboring 0 and 180 degree aperture pairs affected the gate width by 10 nm. The phase of apertures in the PSM should be assigned by taking the phase assignment condition of the neighboring aperture pairs into account when these aperture pairs are placed within the distance of 0.5 micrometers . The CD variation due to defocusing was found to be minimized when the Cr width of fine gate patterns in the PSM was set at 100 nm. The CD variation caused by misalignment between the PSM and the trim mask was also examined, and the trim mask generated by merging the shifter layer gave the minimum CD variation in relation to misalignment. In addition, the CD variation along the length of a fine gate pattern was examined, and the necking shape near the corners of apertures was pointed out. An additional pattern feature was evaluated to avoid it.
The effectiveness of node-connection phase-shifting mask (PSM) was investigated experimentally. In this method, the original design patterns are decomposed into several alternating PSM sub-patterns bas don geometrical rules, and multiple-exposure of these masks reconstructs the original design patterns. This is suitable for random logic interconnects, where wire features are on the 'DA-grid'. We applied this method to patterning a 0.3-micrometers pitch random interconnect with a conventional DUV exposure tool Random interconnect patterns with 0.3-micrometers pitch random interconnect with a conventional DUV exposure tool. Random interconnect patterns with 0.3-micrometers pitch were decomposed using in-house geometrical Boolean operators into three PSMs and were multiply exposed onto the same region of wafer using a KrF exposure tool. Though this is preliminary experiment without mask/process optimization, it shows a possibility of below 0.3-micrometers pitch logic interconnect with KrF tools. Thus, combining this with the phase-edge PSMs or some 'slimming' technologies for shrinking gate-length, the 0.1-micrometers generation logic LSIs are expected to be achieved with conventional DUV exposure tools.
We investigated various exposure procedures to minimize the Critical Dimension (CD) variation for the patterning of sub- quarter micron gates. To examine dependence of the CD variation on the pattern pitch and defocus conditions, the light intensity profiles of four different mask structures: (1) a binary mask with clear field, (2) a binary mask with dark field, (3) a phase-edge type phase-shifting mask (a phase-edge PSM) with clear field, and (4) a halftone phase- shifting mask (a halftone PSM) were compared, where exposure wavelength was 248 nm and numerical aperture (NA) of KrF stepper was 0.55. For 200-nm gate patterns, dependence of the CD variation on the pattern pitch and defocus conditions was minimized by a phase-edge PSM with clear field. By optimizing the illumination condition for a phase-edge PSM exposure, we obtained the CD variation of 10 nm at the minimum gate pitch of 0.8 micrometer and the defocus condition of plus or minus 0.4 micrometer. Applying the optimized exposure procedure to the device fabrication process, we obtained the total CD variation of plus or minus 27 nm.
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