Joost Bekaert

imec Belgium, Louvain, Flanders, Belgium

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Publications (21)0 Total impact

  • [show abstract] [hide abstract]
    ABSTRACT: We studied the potential of optical scatterometry to measure the full 3D profile of features representative to real circuit design topology. The features were selected and printed under conditions to improve the measurability of the features by scatterometry without any loss of information content for litho monitoring and control applications. The impact of the scatterometry recipe and settings was evaluated and optimal settings were determined. We have applied this strategy on a variety of structures and gathered results using the YieldStar angular reflection based scatterometer. The reported results show that we obtained effective decoupling of the measurement of the 3 dimensions of the features. The results match with predictions by calibrated lithographic simulations. As a verification we have successfully performed a scanner matching experiment using computational Pattern Matcher (cPM) in combination with YieldStar as a metrology tool to characterize the difference between the scanners and verify the matching. The results thus obtained were better than using CD-SEM for matching and verification.
    Proc SPIE 03/2012;
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    ABSTRACT: The desire to reduce cost in volume manufacturing has driven up the throughput in the lithographic exposure machines. As a result the power transmitted in the projection optics increases. Although small, the absorption levels in the lens materials are not zero, which leads to localized heating of the lens and hence lens aberrations. To squeeze out the maximum process windows, the pupil shapes have transformed from simple annular shapes to shapes with very concentrated poles. As a result, the exposure energy transported through the lens is no longer equally distributed over the lenses of the projection options. Instead only a fraction of the lens gets to transport the total power. This concentration of power further aggravates the lens heating induced aberrations and enhances the importance of advanced lens heating control schemes which are available on ASML scanners. To analyze the effects of lens heating on the final imaging, a model was developed by the lens manufacturer Carl Zeiss SMT GmbH, and incorporated into a litho simulation environment by ASML BRION. This tool can be used to analyze the impact of dose/throughput, illumination shapes and reticle layout on aberrations. It provides a means to assess potential lens heating issues even before production masks are manufactured. Moreover, this computational tool opens the possibility to calculate parameters for lens heating correction, rather than measuring them, saving valuable machine time. In this paper, the performance of the novel computational lens heating control is demonstrated on wafer and compared with the traditional way of measuring the relevant parameters. In addition, a modeling study is performed to assess possible lens heating effects for freeform or non-traditional source shapes, thereby demonstrating the advanced correction potential of ASML latest aberration manipulator, called FlexWaveTM.
    Proc SPIE 03/2011;
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    ABSTRACT: A negative tone development (NTD) process benefits from the superior imaging performance obtained with light field (LF) masks to print metal and contact layers, resulting in improved process window. In this paper, we introduce an inverse Mack development model to simulate the NTD process and validate its process advantage. Based on this model, a NTD resist model calibration has been carried out and the model results are presented. Various NTD application cases have been studied and the prediction capabilities of simulations are demonstrated: 1) LF+NTD process helps to achieve a broader pitch range and smaller feature size compared to the traditional dark field (DF) with positive tone development (PTD) process. NTD brings a significant improvement in exposure latitude (EL) and MEEF for both line-and-space (L/S) and contact hole (CH) patterns through pitch. 2) The NTD process has been explored for double exposure lithography with extreme off-axis illumination using L/S patterns with horizontal and vertical orientation, respectively, which creates dense contact hole arrays down to a 80 nm pitch. 3) Simulation can also be used to explore new NTD process variances. We have demonstrated the simulations of the NTD model in applications such as printing specific CH or Metal patterns, a dual-tone development process and a combination of source mask optimization (SMO) and NTD to print SRAM patterns at smaller sizes.
    Proc SPIE 03/2011;
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    ABSTRACT: Once a process is set-up in an integrated circuit (IC) manufacturer's fabrication environment, any drift in the proximity fingerprint of the cluster will negatively impact the yield. In complement to the dose, focus and overlay control of the cluster, it is therefore also of ever growing importance to monitor and maintain the proximity stability (or CD through pitch behavior) of each cluster. In this paper, we report on an experimental proximity stability study of an ASML XT:1900i cluster for a 32 nm poly process from four different angles. First, we demonstrate the proximity stability over time by weekly wafer exposure and CD through pitch measurements. Second, we investigate proximity stability from tool-to-tool. In a third approach, the stability over the exposure field (intra-field through-pitch CD uniformity) is investigated. Finally, we verify that proximity is maintained through the lot when applying lens heating correction. Monitoring and maintaining the scanner's optical proximity through time, through the lot, over the field, and from toolto- tool, involves extensive CD metrology through pitch. In this work, we demonstrate that fast and precise CD through pitch data acquisition can be obtained by scatterometry (ASML YieldStarTM S-100), which significantly reduces the metrology load. The results of this study not only demonstrate the excellent optical proximity stability on a XT:1900i exposure cluster for a 32 nm poly process, but also show how scatterometry enables thorough optical proximity control in a fabrication environment.
    Proc SPIE 03/2011;
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    ABSTRACT: IC manufacturers have a strong demand for transferring a working process from one scanner to another. Recently, a programmable illuminator (FlexRayTM) became available on ASML ArF immersion scanners that, besides all the parameterized source shapes of the earlier AerialTM illuminator (based on diffractive optical elements) can also produce any desired freeform source shape. As a consequence, a fabrication environment may have scanners with each of the illuminator types so both FlexRay-to-Aerial and FlexRay-to-FlexRay matching is of interest. Moreover, the FlexRay illuminator itself is interesting from a matching point-of-view, as numerous degrees of freedom are added to the matching tuning space. This paper demonstrates how the upgrade of an exposure tool from Aerial to FlexRay illuminator shows identical proximity behavior without any need for scanner tuning. Also, an assessment of the imaging correspondence between exposure tools each equipped with a FlexRay illuminator is made. Finally, for a series of use-cases where proximity differences do exist, the application of FlexRay source tuning is demonstrated. It shows an enhancement of the scanner matching capabilities, because FlexRay source tuning enables matching where traditional NA and sigma tuning are shortcoming. Moreover, it enables tuning of freeform sources where sigma tuning is not relevant. Pattern MatcherTM software of ASML Brion is demonstrated for the calculation of the optimized FlexRay tuned sources.
    Proc SPIE 03/2011;
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    ABSTRACT: The use of customized illumination modes is part of the pursuit to stretch the applicability of immersion ArF lithography. Indeed, a specific illumination source shape that is optimized for a particular design leads to enhanced imaging results. Recently, freeform illumination has become available through pixelated DOEs or through FlexRayTM, ASML's programmable illuminator system, allowing for virtually unconstrained intensity distribution within the source pupil. In this paper, the benefit of freeform over traditional illumination is evaluated, by applying source mask co-optimization (SMO) for an aggressive use case, and wafer-based verification. For a 22 nm node SRAM of 0.099 mum² and 0.078 mum2 bit cell area, the patterning of the full contact and metal layer into a hard mask is demonstrated with the application of SMO and freeform illumination. In this work, both pixelated DOEs and FlexRay are applied. Additionally, the match between the latter two is confirmed on wafer, in terms of CD and process window.
    Proc SPIE 03/2010;
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    ABSTRACT: A strong demand exists for techniques that can further extend the application of ArF immersion lithography. Besides techniques like litho-friendly design, dual exposure or patterning schemes, customized illumination modes, also alternative processing schemes are viable candidates to reach this goal. One of the most promising alternative process flows uses image reversal by means of a negative tone development (NTD) step with a FUJIFILM solvent-based developer. Traditionally, the printing of contacts and trenches is done by using a dark field mask in combination with positive tone resist and positive tone development. With NTD, the same features can be printed in positive resist using a light field mask, and consequently with a much better image contrast. In this paper, we present an overview of applications for the NTD technique, both for trench and contact patterning, comparing the NTD performance to that of the traditional positive tone development (PTD). This experimental work was performed on an ASML Twinscan XT:1900i scanner at 1.35 NA, and targets the contact/metal layers of the 32 & 22 nm node. For contact hole printing, we consider both single and dual exposure schemes for regular arrays and 2D patterns. For trench printing, we compare the NTD and PTD performance for one-dimensional patterns, line ends and twodimensional structures. We also assess the etch capability and CDU performance of the NTD process. This experimental study proves the added value of the NTD scheme. For contacts and trenches, it allows achieving a broader pitch range and/or smaller litho targets, which makes this process flow attractive for the most advanced lithography applications, including double patterning.
    Proc SPIE 03/2010;
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    ABSTRACT: To ensure defect-free printing, pellicles are mounted on the masks used in optical lithography for IC manufacturing. The pellicle, a thin transparent polymer film, protects the reticle from dust. But, as the 193 nm light transmittance through the pellicle has an angular dependency, the pellicle also acts as an apodization filter. In the current work, we present both experimental and simulation results at 1.35 NA showing the influence of two types of pellicles on proximity and intra-die Critical Dimension Uniformity (CDU) on wafer. The considered structures are compatible with the 32 nm logic node for poly and metal. For the standard ArF pellicle (thickness 830 nm), we experimentally observe a distinct effect of several nm's of the pellicle presence on both the proximity and the intra-die CDU. For the more advanced pellicle (280 nm thin) no signature of the pellicle on proximity or CDU could be found. By modeling the pellicle's optical properties as a Jones Pupil, we are able to simulate the pellicle effects with good accuracy. These results indicate that for the 32 nm node, it is recommended to take the pellicle properties into account in the OPC calculation when using a standard pellicle. Simulations also indicate that, in addition to that, a local dose correction can compensate to a large extent for the intra-die pellicle effect. When using the more advanced thin pellicle (280 nm), no such corrections are needed.
    Proc SPIE 03/2010;
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    ABSTRACT: Several options are being explored to extend device scaling towards and beyond the 32nm Half Pitch (HP) using the current immersion lithography tools and this in order to compete with the costly EUV technology that is still under development. These extension techniques all involve compromises between design and process. In this paper, several options for the extension beyond the 32nm HP node are investigated and illustrated with experimental results. In a first stage, a litho-friendly design is created, enabling the scalability by lithography. Secondly, aerial image contrast and pitch can be pushed to the ultimate limits by splitting the design into two masks. One mask contains horizontal features and the other one vertical features and both will be printed with extreme off-axis illumination. Double Patterning (DP) is the next step which enables pitch scaling beyond the limits of 1.35NA exposures. The most common double patterning technique used is litho-etch-litho-etch. A splitted design is recombined through two subsequent patterning steps. Self- Aligned Double Patterning is another pitch doubling technique, interesting for one-dimensional designs on narrow pitches. Next to it, alternative, more cost effective DP approaches are discussed. These techniques show the capability of immersion lithography and double patterning to scale beyond the 32nm HP node.© (2009) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
    10/2009;
  • [show abstract] [hide abstract]
    ABSTRACT: Several options are being explored to extend device scaling towards and beyond the 32nm Half Pitch (HP) using the current immersion lithography tools and this in order to compete with the costly EUV technology that is still under development. These extension techniques all involve compromises between design and process. In this paper, several options for the extension beyond the 32nm HP node are investigated and illustrated with experimental results. In a first stage, a litho-friendly design is created, enabling the scalability by lithography. Secondly, aerial image contrast and pitch can be pushed to the ultimate limits by splitting the design into two masks. One mask contains horizontal features and the other one vertical features and both will be printed with extreme off-axis illumination. Double Patterning (DP) is the next step which enables pitch scaling beyond the limits of 1.35NA exposures. The most common double patterning technique used is litho-etch-litho-etch. A splitted design is recombined through two subsequent patterning steps. Self- Aligned Double Patterning is another pitch doubling technique, interesting for one-dimensional designs on narrow pitches. Next to it, alternative, more cost effective DP approaches are discussed. These techniques show the capability of immersion lithography and double patterning to scale beyond the 32nm HP node.
    Proc SPIE 10/2009;
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    ABSTRACT: Contact Hole (CH) resolution is limited by the low aerial image contrast using dark field masks. Moreover the 2- Dimensional character of CH is a limiting factor in the use of extreme Resolution Enhancement Techniques for reaching the smallest pitch. These limitations can be overcome if one deconvolves the 2D CH into two exposures of 1D structures (i.e. lines). These 1D structures can indeed be printed at the ultimate resolution limit of the scanner using dipole exposures. Recently, several materials have become available to pattern CH from such a double exposure of line patterns. It is shown in this paper how this concept of deconvolution can be used in different techniques: Two 1D aerial images can be recomposed in order to obtain 2D images which will subsequently be reversed into CH. We can distinguish, on the one hand, a reversal based on the positive development of line crossings into resist pillar patterns, on which are deposited or coated a gap-fill material layer. The pillars are then removed, leaving a masking material layer with holes. On the other hand, negative tone development can be used to reverse directly the recomposed 2D aerial image: while the classical positive development creates pillars, the negative tone development inverses immediately this image to create contact holes in the resist layer. In this paper, we demonstrate the potential of the double exposure method. We characterise three reversal techniques using a NA=1.35 immersion scanner for patterning 40nm or lower CH at pitch 80nm. We also show etch performance of these processes and address the complexity of each solution.
    Proc SPIE 03/2009;
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    ABSTRACT: IC manufacturers have a strong demand for transferring a working process from one scanner to another. In an ideal transfer, a reticle set that produces devices within specification on a certain scanner has the same performance on another exposure tool. In real life, however, reticles employ optical proximity correction (OPC) which incorporates by definition the inherent optical fingerprint of a specific exposure tool and process. In order to avoid the additional cost of developing a new OPC model and acquiring a new reticle for each exposure tool, IC manufacturers therefore wish to 'match' the optical fingerprint of their scanners as closely as possible. In this paper, we report on the matching strategy that we developed to perform a tool-to-tool matching. We present experimental matching results for several tool combinations at numerical apertures (NA) 0.75, 0.85 and 1.2. Matching of exposure tools is obtained by determining the sensitivities to scanner parameter variations like NA, Sigma, Focus Drilling, Ellipticity and Dose from wafer data and/or simulations. These sensitivities are used to calculate the optimal scanner parameters for matching the two tools.
    Proc SPIE 11/2008;
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    ABSTRACT: The ever increasing NA to meet the stringent requirements on pitch and litho target does not come without a price. In particular for the printing of contact layers, the toll is taken in terms of decreased process latitudes and CD uniformity. Depending on the specific contact layer design, it has become increasingly important to choose the most adequate resolution enhancement technique. At the ultimate NA of 1.35, depth of focus through pitch has become a key factor, and values in the order of 100-120nm are the painful truth imposed by physics. In this paper, the attention goes to through-pitch contact imaging, with the attempt to achieve half pitches around 60nm. Thereby, the main focus lies on the relation between source shape and minimum achievable k1. In addition, the pro's and con's of two options for throughpitch process latitude enhancement are considered. These options are firstly the effect of assist feature placement in combination with off-axis illumination, and secondly the application of the Focus Drilling technique. Finally, the different contributions to contact hole CD non-uniformity are addressed.
    Proc SPIE 03/2008;
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: IC manufacturers have a strong demand for transferring a working process from one scanner to another. In an ideal transfer, a reticle set that produces devices within specification on a certain scanner has the same performance on another exposure tool. In real life, however, reticles employ optical proximity correction (OPC) which incorporates by definition the inherent optical fingerprint of a specific exposure tool and process. In order to avoid the additional cost of developing a new OPC model and acquiring a new reticle for each exposure tool, IC manufacturers therefore wish to "match" the optical fingerprint of their scanners as closely as possible. In this paper, we report on the matching strategy that we developed to perform a tool-to-tool matching. We present experimental matching results for several tool combinations at numerical apertures (NA) 0.75, 0.85 and 1.2. Matching of two exposure tools is obtained by determining the sensitivities to scanner parameter variations like NA, Sigma, Focus Drilling, Ellipticity and Dose from wafer data and/or simulations. These sensitivities are used to calculate the optimal scanner parameters for matching the two tools.
    Proc SPIE 03/2008;
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    ABSTRACT: Double patterning technology (DPT) is a promising technique that bridges the anticipated technology gap from the use of 193nm immersion to EUV for the half-pitch device node beyond 45nm. The intended mask pattern is formed by two independent patterning steps. Using DPT, there is no optical imaging correlation between the two separate patterning steps except for the impact from mask overlay. In each of the single exposure step, we can relax the dense design pattern pitches by decomposing them into two half-dense ones. This allows a higher k1 imaging factor for each patterning step. With combined patterns, we can achieve overall k1 factor that exceeds the conventional Rayleigh resolution limit. This paper addresses DPT application challenges with respect to both mask error factor (MEF) and 2D patterning. In our simulations using DPT with relaxed feature pitch for each exposure step, the MEF for the line/space is fairly manageable for 32nm half-pitch and below. The real challenge for the 32nm half-pitch and below with DPT is how to deal with the printing of small 2D features resulting from the many cutting sites due to feature decomposition. Each split of a dense pattern generates two difficult-to-print line-end type features with dimension less than one-fifth or one-sixth of ArF wavelength. Worse, the proximity environment of the 2D cut features can then become quite complex. How to stitch them correctly back to the original target requires careful attention. Applying target bias can improve the printing performance in general. But using a model-based stitching error correction method seems to be a preferred solution.
    Proc SPIE 10/2006;
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    ABSTRACT: Patterning of random CH for the 65nm node and below has proven to be a very difficult task. As a rule of thumb, difficulties in contact patterning are driven by the low depth of focus towards isolated contacts and/or the lower contrast combined with higher mask error factor (MEEF) for denser contact arrays. In this work, we experimentally investigate the use of illumination modes consisting of the combination of annular plus conventional illumination, so-called "Bulls-Eye" illumination. This study is a search for the optimal sigma settings for the annular and conventional parts, with respect to process window and MEEF through pitch. Also, the extend to which the Bulls-Eye is advantageous is demonstrated by means of experimental comparison to wafer prints by conventional illumination. Besides regular grid CH arrays, the Bulls-Eye performance is evaluated for different 2D contact patterns. Experimental results are obtained on ASML ArF scanners at various NAs up to 0.93. Additionally, immersion lithography and Focus Drilling are considered at given exposure setting as techniques to increase the focal depth. The experiments show promising results, printing contacts from k1 = 0.40 onwards with acceptable process, MEEF, and proximity through pitch, and this without side-lobe printing.
    Proc SPIE 04/2006;
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    ABSTRACT: Immersion lithography offers the semiconductor industry an opportunity to extend the current ArF processes to smaller nodes before switching to a shorter wavelength. The transition to immersion will require increased attention to the photomask along with new effects influencing the aerial image formation as the numerical apertures (NA) of scanners move up to at least 0.93 and beyond. Feature sizes on the photomask become comparable to, or even smaller than the wavelength and hence act more like wire grid polarisers which lead to polarisation effects. As of today AIMSTM fab tools are in operation worldwide, with the novel AIMSTM fab 193i offering a maximum NA of 0.93 and is the latest aerial image measurement system for ArF-lithography emulation down to the 65nm node. Common adjustments include numerical aperture, illumination type and partial illumination coherence to match the conditions in 193nm scanners. In addition to unpolarised illumination, the AIMSTM fab 193i allows the user to select linear x and y polarised light for different settings and types, e.g. off-axis annular, quadrupole or dipole illumination. In this paper the polarisation effects of different photomask features are explored by comparing measurement results using linear polarised illumination parallel and perpendicular to line and space patterns and non-polarised illumination. A new scanner mode will be presented for the investigation of contrast loss due to polarisation effects from imaging.
    Proc SPIE 04/2006;
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    ABSTRACT: Patterning of random CH for the 65nm node and below has proven to be a very difficult task. As a rule of thumb, difficulties in contact patterning are driven by the low depth of focus towards isolated contacts and/or the lower contrast combined with higher mask error factor (MEEF) for denser contact arrays. In this work, we experimentally investigate the use of illumination modes consisting of the combination of annular plus conventional illumination, so-called "Bulls-Eye" illumination. This study is a search for the optimal sigma settings for the annular and conventional parts, with respect to process window and MEEF through pitch. Also, the extend to which the Bulls-Eye is advantageous is demonstrated by means of experimental comparison to wafer prints by conventional illumination. Besides regular grid CH arrays, the Bulls-Eye performance is evaluated for different 2D contact patterns. Experimental results are obtained on ASML ArF scanners at various NAs up to 0.93. Additionally, immersion lithography and Focus Drilling are considered at given exposure setting as techniques to increase the focal depth. The experiments show promising results, printing contacts from k1 = 0.40 onwards with acceptable process, MEEF, and proximity through pitch, and this without side-lobe printing.© (2006) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
    03/2006;
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    ABSTRACT: Different types of phase shifting masks in combination with the proper illumination condition are widely used to allow 193nm lithography to print ever-decreasing pitches with sufficient process window. A viable option for the 65nm and 45nm node is chromeless phase lithography (CPL), which combines a chromeless phase shift mask and 193nm off-axis illumination. Previously, we demonstrated that imaging-wise the pi-shift is not entirely reached in narrow mask features, although etched to the nominal etch depth. This is caused by the mask 3D effect, and manifests itself in through focus Bossung tilt/shift issues. In particular, 3D mask simulations suggested that an effective pi phase shift could be recovered for the narrow chromeless mask features by applying a larger than nominal etch depth. In this work, the applicability of this solution is considered in more detail. Experimental through-pitch solutions for regular line/space patterns using CPL, obtained on three latest generations of ASML ArF scanners: 0.75NA (PAS5500 /1100), 0.85NA (XT:1250Di), and 0.93NA (XT:1400i) are demonstrated. Importantly, it is illustrated that mask etch depth adjustment is a widely applicable practical solution to the CPL Bossung tilt/shift issue. The effect of source shape, increasing NA, as well as the effect of immersion versus 'dry' lithography is evaluated. Additionally, the ultimate resolution limits at 0.93NA (XT:1400) are explored. Data obtained on a Zeiss AIMS fab193i shows to be in line with both the exposure data and the 3D simulated data, confirming the clear reduction of focus tilt/shift when using the larger than nominal etch depth.
    Proc SPIE 11/2005;
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    ABSTRACT: Chromeless-Phase Lithography (CPL) combined with IML (Interference Mapping Lithography) technology is experimentally demonstrated as a viable resolution enhancement technique (RET) to pattern low-k1 (0.39) contact holes (CHs) from dense through sparse pitches. Both the process latitude and the MEEF values are measured. The most promising single exposure techniques combine off-axis-illumination (OAI) with the use of non-printing assist features, as in the case of CPL with IML. Contrary to other RETs, CPL does not use sub-resolution assist features but non-printing assist slots with a well-chosen phase (180° or 0°) and transmission (0% or 100%) assignment. The optimization and the positioning of assist features result of IML, based on a mapping of the field intensity at the wafer level: the assist features interfere to enhance the image at the contact hole location. The experimental layout optimization is discussed, showing how the process is maximized together with the dose-margin before any side-lobe printing. Using ArF immersion lithography at 0.75 NA with Quasar 20° sigmaout=0.92 / sigmain=0.72, the CPL printing performance of 100 nm contact holes, from 200 nm pitch through isolated, is measured. The Depth-Of-Focus at 8% Exposure Latitude (DOF @ 8% EL) remains above 0.4 mum through pitch, with 0.43 mum DOF @ 8% EL at the difficult 300 nm pitch. The MEEF becomes a multi-dimensional metric on CPL masks. The wafer CD uniformity depends not only on the size variation of the CH on the reticle, but also on the size variations at the two reticle write steps, i.e. the assist slots opening and the Cr removal. The MEEF metrics related to the CH and slot sizes appear as the most critical ones. For each of those parameters, measured MEEF is always below 3.
    Proc SPIE 08/2004;