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Modeling for profile-based process-window metrology
Abstract
We formulate a physical model to extract effective dose and defocus (EDD) from pattern profile data and demonstrate its efficacy in the analysis of in-line scatterometer measurements. From the measurement of a single target structure, the model enables simultaneous computation of pattern dimensions pre-calibrated to the imaging system dose and focus settings. Our approach is generally applicable to ensuring the adherence of pattern features to dimensional tolerances in the control and disposition of product wafers while minimizing in-line metrology.
... The measurement of focus using scatterometry is based on the well known lithographic principle of profile variation through focus as previously published by others [8,9,10]. Since this relation is dependent on the exact lithographic conditions like the target characteristics, the scanner illumination conditions and the process, there is a need for a application specific calibration. ...
... The key element in the focus and dose measurement performance is enclosed in the target design. In contrast to other published strategies [8][9][10][11][12], we have chosen to optimize the focus/dose targets for maximum focus sensitivity and maximum flexibility towards its usability for a range of illumination settings. ...
... The second step is actually an inversion of the Bossung curves measured in the first step. Ausschnitt et al. [10] propose to model the Bossung curves using a 2 nd order function. This approximation leads to a closed form analytical solution for the inverse problem. ...
Holistic lithography is needed to cope with decreasing process windows and is built on three pillars: Scanner Tuning, Computational Lithography and Metrology & Control. The relative importance of stability to the overall manufacturing process latitude increases. Overlay and focus stability control applications are important elements in improving stability of the lithographic process. The control applications rely on advanced control algorithms and fast and precise metrology. To address the metrology needs at the 32 nm node and beyond, an optical scatterometry tool was developed capable of measuring CD, focus-dose as well as overlay. Besides stability and control of lithographic performance also scanner matching is a critical enabler where application development and metrology performance are key. In this paper we discuss the design and performance of the metrology tool, the focus and overlay control application and the application of scatterometry in scanner matching solutions.
... Scatterometry has the significant benefits of fast, accurate, repeatable, and non-destructive measurement of the critical dimension, film thickness, and profile shape. Considerable research has been done on the scatterometry based focus and exposure monitoring techniques [3][4][5][6]. The general method involves using scatterometry to measure resist profile, and enabling independent solutions for dose and focus by fitting a semi-physical model to the data. ...
... As a focus monitor, a linear model is easily developed in Eq.1. 3 ...
This paper proposes a new highly sensitive scatterometry based Probe-Pattern Grating Focus Monitor. The high sensitivity is achieved by placing transparent lines spaced at the strong focus spillover distance of around 0.6λ/NA from the centerline of a 90 degree phase-shifted probe line that functions as an interferometer detector. The monitor translates the focus error into the probe line trench depth, which can be measured by scatterometry techniques. Simulations of optical imaging, resist development and Optical Digital Profilometry measurements are used to evaluate the expected practical performance. A linear model is developed to estimate focus error based on the measured probe trench depth. The results indicate that the ODP measurement from a single wafer focus setting can detect both the defocus direction and the defocus distance to well under 0.1 Rayleigh unit of defocus.
... The model of scatterometry technique is10 ...
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.
... The methodology used for calculating dose and focus information from scatterometry CD measurements is that reported by Ausschnitt and Cheng in 2004 4 . This method involves certain simplifying assumptions that are dealt with in the referenced paper and not here. ...
Focus and dose control of lithography tools for leading edge semiconductor manufacturing are critical to obtaining acceptable process yields and device performance. The need for these controls is increasing due to the apparent limitation of optical water immersion lithography at NA values of approximately 1.35 and the need to use the same equipment for 45nm, 32nm, and 22nm node production. There is a rich history of lithographic controls using various techniques described in the literature. These techniques include (but are not limited to) Phase Grating Focus Monitoring1 (PGFM), optical CD control using optical overlay metrology equipment (OOCD)2,3, and in more recent years optical scatterometry4,5. Some of the techniques, even though they are technically sound, have not been practical to implement in volume manufacturing as controls for various reasons. This work describes the implementation and performance of two of these techniques (optical scatterometry and OOCD) in a volume 300mm production facility. Data to be reviewed include: - General implementation approach. - Scatterometry dose and focus stability data for 193nm immersion and 248nm dry lithography systems. - Analysis of the stability of optical scatterometry dose and focus deconvolution coefficients over time for 193nm immersion and 248nm dry systems. - Comparison between scatterometry and OOCD techniques for focus monitoring of 248nm dry systems. The presentation will also describe the practical issues with implementing these techniques as well as describe some possible extensions to enhance the current capabilities being described.
... Empirical estimators can also be built for other semiconductor manufacturing process variables and other properties of interest, including the electrical characteristics of the circuit being built. New examples of such capabilities for CD-SEM and optical [15,16] metrology tools continue to emerge. ...
CD-SEM is the metrology tool of choice for patterning process development and production process control. We can make these applications more efficient by extracting more information from each CD-SEM image. This enables direct monitors of key process parameters, such as lithography dose and focus, or predicting the outcome of processing, such as etched dimensions or electrical parameters. Automating CD-SEM recipes at the early stages of process development can accelerate technology characterization, segmentation of variance and process improvements. This leverages the engineering effort, reduces development costs and helps to manage the risks inherent in new technology. Automating CD-SEM for manufacturing enables efficient operations. Novel SEM Alarm Time Indicator (SATI) makes this task manageable. SATI pulls together data mining, trend charting of the key recipe and Operations (OPS) indicators, Pareto of OPS losses and inputs for root cause analysis. This approach proved natural to our FAB personnel. After minimal initial training, we applied new methods in 65nm FLASH manufacture. This resulted in significant lasting improvements of CD-SEM recipe robustness, portability and automation, increased CD-SEM capacity and MT productivity.
... After creating a baseline overlay and/or focus fingerprint, the control algorithm is capable of recovering the scanner performance back to this initial baseline fingerprint after drift in the scanner or periodic maintenance type interventions occurred with minimal impact. In Figure 2 the monitor wafer flow (2) in the BaseLiner application is depicted in the context of other running processes. The monitor wafers are exposed and measured every 2-3 days. ...
As leading edge lithography moves to 22-nm design rules, low k1 technologies like double patterning are the new resolution enablers, and system control and setup are the new drivers to meet remarkably tight process requirements. The way of thinking and executing setup and control of lithography scanners is changing in four ways. First, unusually tight process tolerances call for very dense sampling [1], which in effect means measurements at high throughput combined with high order modeling and corrections to compensate for wafer spatial fingerprint. Second, complex interactions between scanner and process no longer allow separation of error sources through traditional metrology approaches, which are based on using one set of metrology tools and methods for setup and another for scanner performance control. Moreover, setup and control of overlay is done independently from CD uniformity, which in effect leads to independent and conflicting adjustments for the scanner. Third, traditional CD setup and control is based on the focus and dose calculated from their CD response and not from measurement of their effect on pattern profile, which allows a clean and orthogonal de-convolution of focus and dose variations across the wafer. Fourth, scanner setup and control has to take into consideration the final goal of lithography, which is the accurate printing of a complex pattern describing a real device layout. To this end we introduce a new setup and control metrology step: measuring-to-match scanner 1D and 2D proximity. In this paper we will describe the strategy for setup and control of overlay, focus, CD and proximity based on the YieldStarTM metrology tool and present the resulting performance. YieldStar-200 is a new, high throughput metrology tool based on a high numerical aperture scatterometer concept. The tool can be used stand-alone as well as integrated in a processing track. It is suitable for determining process offsets in X,Y and Z directions through Overlay and Focus measurements respectively. In addition CD profile information can be measured enabling proximity matching applications. By using a technique [2][3][4] to de-convolve dose and focus based on the profile measurement of a well-characterized process monitor target, we show that the dose and focus signature of a high NA 193nm immersion scanner can be effectively measured and corrected. A similar approach was also taken to address overlay errors using the diffraction based overlay capability [5] of the same metrology tool. We demonstrate the advantage of having a single metrology tool solution, which enables us to reduce dose, focus and overlay variability to their minimum non-correctable signatures. This technique makes use of the high accuracy and repeatability of the YieldStar tool and provides a common reference of scanner setup and user process. Using ASML's YieldStar in combination with ASML scanners, and control solutions allows for a direct link from the metrology tool to the system settings, ensuring that the appropriate system settings can be easily and directly updated.
... This method does show promising focus sensitivity, but it requires special test structures and does not provide the precise magnitude and direction of change information that would be required for making immediate corrections. Another method involves using scatterometry data to fit FEM models for two different points on the resist profile, thereby enabling independent solutions for dose and focus 10 . It is this general method that we will expound upon in this paper. ...
In this paper, results and analysis are presented from Advanced Micro Devices' (AMD) efforts at calculating lithography dose and focus parameters using scatterometry metrology and semi-physical CD models. The system takes advantage of the accurate and precise top and bottom CD data produced by scatterometry to differentiate dose and focus variation. To build the lithography process model, scatterometry data is generated for each field of a focus-exposure matrix (FEM) wafer, and the resulting top and bottom CD data is used to fit the parameters of series expansions relating CD to dose and focus. When new CD data is generated, the models can be inverted to solve for dose and focus independently. Our methodology employs a flexible modeling and inversion approach in an attempt to make the technique applicable to any production film stack and any line spacing regime. The quality of the inversion results are highly correlated to the degree of focus observability present in the system. Our results will show how a series of litho process with varied film stacks and line/space ratios respond to this technique, and we will report some best practices for a variety of use cases ranging from equipment characterization to focus monitoring on product.
... Consider now the common application of the process window analysis commonly embodied by equation 6: [4] Where we now describe the feature response as that for a measured entity located at position (x,y) on the exposure field. The variables involved in this equation use the common definitions of E as the exposure dose, Es being the nominal dose to achieve image size and F being the defocus of the exposure. ...
It's commonly reported that a difference exists between directly measured reticle feature dimensions and those produced in the final lithographic image. Quantifying this mask error function (MEF) and the sources of the perturbation has been the topic of many papers of the past several years. Past studies have been content to evaluate these functions by statistical averaging thereby neglecting the potential influence of process and exposure contributions. The material presented here represents the findings of an extensive study of reticle-process interactions. Phase I of the evaluation consisted of focus and dose exposures of the reticle and subsequent modeling of the full-profile response. This analysis provided extensive information on the optimum-printed feature profiles while removing the contribution of across-field focus variations. The reticle was directly characterized using both conventional SEM and a new Nanometrics OCD Scatterometer technique. The full-field modeled response surface of the directly measured feature characteristics are then used to calculate the across-field MEF and provide an improved estimate of the true response of the feature to exposure. Phase II of the analysis turns its attention to characterization of the full-wafer process response. Both the modeled and directly measured reticle surfaces were removed from Scatterometry measured full-wafer exposures. Normal process variations consisting of photoresist and ARC thickness volatility are next used to show the response of the printed feature. Finally a summary of the relative contribution of each process perturbation to the feature profile error budget is discussed.
... Nanometrics' ATLAS TM supports spectroscopic-ellipsometer which is used to measure blanket wafers for optical constant determination. The advantages of OCD measurement over traditional CDSEM measurement in CD and profile monitoring have been demonstrated extensively 5,6,7 . Some of the advantages are particularly important for OPC modeling: ...
The ability to manage critical dimensions (CDs) of structures on IC devices is vital to improving product yield and performance. It is challenging to achieve accurate metrology data as the geometries shrink beyond 40 nm features. At this technology node CDSEM noise and resist LER are of significant concerns1. This paper examines the extendibility of scatterometry techniques to characterize structures that are close to limits of lithographic printing and to extract full profile information for 2D and 3D features for OPC model calibration2. The resist LER concerns are diminished because of the automatic averaging that scatterometry provides over the measurement pad; this represents a significant added value for proper OPC model calibration and verification. This work develops a comparison matrix to determine the impact of scatterometry data on OPC model calibration with conventional CDSEM measurements. The paper will report test results for the OPC model through process data for accuracy and predictability.
... One promising method for improved focus monitoring involves the use of a series expansion published by Mack and Byers relating the CD to changes in dose and focus, 1 and a profile-based method of deconvoluting dose and focus published by Ausschnitt and Cheng. 2 A series of implementation methodologies for this technique have been published involving different approaches to truncating the series expansion, and special measurement targets for maximizing sensitivity to focus. 3 To date, most work has been based on scatterometry metrology, but any data source may be used that is capable of producing two different CD variables for a single feature that are not strongly correlated in their response to dose and focus. ...
We present a summary of various methods for inverting top and bottom critical dimension (CD) data to extract dose and focus information. We explain analytical, numerical, and library inversion techniques in detail, and explore their relative merits for the purposes of online and offline focus monitoring use models. We also detail the modeling requirements associated with each inversion technique, and -- for cases where the model form is flexible -- present a cross-validation methodology for optimizing the response model to fit experimental data. We present modeling and inversion results from seven exemplary photolithography processes, and study the results from each methodology in detail. While each method has its own set of advantages and disadvantages, we show that the library method represents the optimum choice to satisfy a variety of use models while minimizing cost.
... There are a number of reports in the literature about extracting focus and exposure conditions from the profile of features printed in resist. [1][2][3][4] We will restrict our discussion of this technique to its basic principles, which are illustrated in Next, we conducted an experimental comparison of this SFDM technique with the phase-grating focus monitoring (PGFM) technique, on which we rely for our most critical focus evaluation needs. Wafers were printed with an array of focus and exposure dose conditions, also known as a focus-exposure matrix (FEM), and we extracted focus values from the SFDM and the PGFM techniques. ...
Scatterometry techniques are used to characterize the CD uniformity, focus and dose control, as well as the image contrast of a hyper-NA immersion lithography scanner. Results indicate very good scanner control and stability of these parameters, as well as good precision and sensitivity of the metrology techniques.
... Subsequent sparse sampling of the features in the lot during the production cycle allowed the inverse model to be solved for localized exposure and focus. [3] Significant work continues in this area with other publications directed toward techniques for reducing the subjective nature involved in selecting the proper coefficients or weighting strategies to include in the fit or by pre-conditioning the equations with simulations. [4] One of the more recent renditions of the process window algorithm consists of an expansion in focus and dose of the form: * Contact: tzavecz@TEAsystems.com ...
Most process window analysis applications are capable of deriving the functional focus-dose workspace available to any set of device specifications. Previous work in this area has concentrated on calculating the superpositioned optimum operating points of various combinations of feature orientations or feature types. These studies invariably result in an average performance calculation that is biased by the impact of the substrate, reticle and exposure tool contributed perturbations. Many SEM's and optical metrology tools now provide full-feature profile information for multiple points in the exposure field. The inclusion of field spatial information into the process window analysis results in a calculation of greater accuracy and process understanding because now the capabilities of each exposure tool can be individually modeled and optimized. Such an analysis provides the added benefit that after the exposure tool is characterized, it's process perturbations can be removed from the analysis to provide greater understanding of the true process performance. Process window variables are shown to vary significantly across the exposure field of the scanner. Evaluating the depth- of-focus and optimum focus-dose at each point in the exposure field yields additional information on the imaging response of the reticle and scan-linearity of the exposure tool's reticle stage. The optimal focus response of the reticle is then removed from a full wafer exposure and the results are modeled to obtain a true process response and performance.
... Scatterometry has the significant benefits of fast, accurate, repeatable, and non-destructive measurement of the profile shape, critical dimension, and film thickness. Considerable research has been done on the scatterometry-based focus and exposure monitoring techniques [3][4][5][6][7]. The general method involves fitting a semi-physical model to the resist profile through scatterometry measurement of the CDs, side wall angle, and etc., then solves the inverse model to determine the exposure conditions. ...
This paper presents a new highly sensitive scatterometry based Probe-Pattern Grating (PPG) focus monitor and its printing assessment on an advanced exposure tool. The high sensitivity is achieved by placing transparent lines spaced at the strong focus spillover distance from the centerline of a 90 degree phase-shifted probe line that functions as an interferometer detector. The monitor translates the focus error into the probe line trench depth, which can be measured by scatterometry techniques. The sensitivity of the defocus measurement through scatterometry calibration is around 1.1nm defocus / nm trench depth. This result indicates that the PPG focus monitor from a single wafer focus setting can detect the defocus distance to well under 0.05 Rayleigh Units.
... An algorithmic approach to run-to-run control of scanner focus and dose using in-line metrology was presented by Ausschnitt and Cheng [3]. This methodology independently calibrated the focus-dose control surface of the exposure tool for both profile top and bottom measurements using the inverted dose model of equation <1: ...
Competitive high volume semiconductor manufacturing yields require that critical feature profiles be continually monitored for uniformity and production control. Historically this has involved long and tedious analyses of Scanning Electron Microscope (SEM) photos that resulted in an average feature profile or a qualitative comparison of a matrix of black and white images. Many factors influence profiles including wafer flatness, focus and film thicknesses. Characterizing profile uniformity as a function of these parameters not only stabilizes high product yields but also significantly reduces the time spent in problem aversion and solution discovery. Scatterometry uniquely provides the combination of feature metrics and spatial coverage needed to monitor production profiles. The vast amount of data gathered by these systems is not well handled by classic statistical methods. A more practical approach taken by the authors is to apply spatial models to the profile data to determine the relative stability and contributions of film, substrate and the exposure tool to process perturbations. Recent work performed by Agere and TEA Systems is shown to be capable of quantitatively modeling the relative contributions of lens slit, reticle-scan and lens degradation to feature size and side-wall angle (SWA). This work describes the models used and the slit-and-scan contributions that are unique for each exposure tool. Finally it is shown that the direction and linearity of the reticle scan can be a contributing factor to the feature profile error budget with direct influence production image stability.
In recent technology nodes, advanced process and novel integration scheme have challenged the precision limits of conventional metrology; with critical dimensions (CD) of device reduce to sub-nanometer region. Optical metrology has proved its capability to precisely detect intricate details on the complex structures, however, conventional RCWA-based (rigorous coupled wave analysis) scatterometry has the limitations of long time-to-results and lack of flexibility to adapt to wide process variations. Signal Response Metrology (SRM) is a new metrology technique targeted to alleviate the consumption of engineering and computation resources by eliminating geometric/dispersion modeling and spectral simulation from the workflow. This is achieved by directly correlating the spectra acquired from a set of wafers with known process variations encoded. In SPIE 2015, we presented the results of SRM application in lithography metrology and control [1], accomplished the mission of setting up a new measurement recipe of focus/dose monitoring in hours. This work will demonstrate our recent field exploration of SRM implementation in 20nm technology and beyond, including focus metrology for scanner control; post etch geometric profile measurement, and actual device profile metrology.
Focus monitoring on actual product can be implemented with high sensitivity if the appropriate structure is chosen. One possible metric is the difference between bottom and top widths of isolated lines. To ensure successful deployment, process engineers should employ noise-reduction techniques and CD-SEM matching. Feature orientations and pitches make it possible to identify tool problems such as changing illumination conditions and lens aberrations.
CD uniformity requirements at 20nm and more advanced nodes have challenged the precision limits of CD-SEM metrology, conventionally used for scanner qualification and in-line focus/dose monitoring on product wafers. Optical CD metrology has consequently gained adoption for these applications because of its superior precision, but has been limited adopted, due to challenges with long time-to-results and robustness to process variation. Both of these challenges are due to the limitations imposed by geometric modeling of the photoresist (PR) profile as required by conventional RCWA-based scatterometry. Signal Response Metrology (SRM) is a new technique that obviates the need for geometric modeling by directly correlating focus, dose, and CD to the spectral response of a scatterometry tool. Consequently, it suggests superior accuracy and robustness to process variation for focus/dose monitoring, as well as reducing the time to set up a new measurement recipe from days to hours. This work describes the fundamental concepts of SRM and the results of its application to lithography metrology and control. These results include time to results and measurement performance data on Focus, Dose and CD measurements performed on real devices and on design rule metrology targets.
In this work, we report the first demonstration of scatterometry Optical Critical Dimension (OCD) characterization on advanced Ge Multi-Gate Field-Effect Transistor (MuGFET) or FinFET formed on a Germanium-on-Insulator (GeOI) substrate. Two critical process steps in the Ge MuGFET process flow were investigated, i.e. after Ge Fin formation, and after TaN gate stack etching process. All key process variations in the test structures were successfully monitored by the floating or fitting parameters in the OCD models. In addition, excellent static repeatability, with 3σ lower than 0.12 nm, was also achieved. The measurement results from OCD were also compared with both Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) measurements. Excellent correlation with both SEM and TEM was achieved by employing OCD characterization, confirming scatterometry OCD as a promising metrology technique for next generation multi-gate transistor with an advanced channel material.
In this work, we report metrology solutions using scatterometry Optical Critical Dimension (OCD)
characterization on two advanced CMOS devices: novel n-channel gate-last In0.53Ga0.47As FinFET with
self-aligned Molybdenum (Mo) contacts and p-channel Ge FinFET formed on Germanium-on-Insulator
(GOI) substrate. Key critical process steps during the fabrication of these advanced transistors were
identified for process monitor using scatterometry OCD measurement to improve final yield. Excellent
correlation with reference metrology and high measurement precision were achieved by using OCD
characterization, confirming scatterometry OCD as a promising metrology technique for next generation
device applications. In addition, we also further explore OCD characterization using normal incidence
spectroscopic reflectometry (SR), oblique incidence spectroscopic ellipsometry (SE), and combined SR+SE
technologies. The combined SR+SE approach was found to provide better precision.
We introduced a simple method based on scatterometry measurement
performed on dense contact holes matrix to investigate intrafield focus
deviation on 28nm FDSOI real production wafers at contact holes
patterning lithography operation. A complex three-dimensional
scatterometry model with all patterned resist geometrical parameters
left as degree of freedom. Then simple linear relationships between
patterned resist geometrical parameters on the one hand, and applied
dose and focus offset on the other hand were used to determine a focus
and dose decorrelation model. This model was then used to investigate
the effect of ASML AGILETM scanner option on intrafield focus
deviation. A significant 16% intrafield focus standard deviation
improvement was found with AGILETM, which validated our
method and shows the possibilities of AGILETM option for
intrafield focus control. This focus investigation method may be used to
improve advanced CMOS manufacturing process control.
High volume semiconductor manufacturing yields require that critical
resist feature profile is continually controlled for uniformity and
centering. One reason is the small working distance of high numerical
aperture lenses. Indeed, reducing process windows require more precise
dimensional control. The variation of the critical dimensions can
generally be attributed to the lack of the focus and/or dose control. A
methodology to control the two lithographic parameters and to construct
a focus and dose budget for all components (tool, layer, resist, and
reticle) has been developed. This paper presents a run-to-run control
called FDO1 (Focus Dose Optimization) using in-line CD metrology. We
have confirmed that this method controls the photoresist shape and the
photoresist width accurately and reduces the CD variation for 28 nm
devices by 50%.
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.
This work describes the implementation and performance of AGILE focus corrections for advanced photo lithography in volume production as well as advanced development in IBM's 300mm facility. In particular, a logic hierarchy that manages the air gage sub-system corrections to optimize tool productivity while sampling with sufficient frequency to ensure focus accuracy for stable production processes is described. The information reviewed includes: General AGILE implementation approaches; Sample focus correction contours for critical 45nm, 32nm, and 22nm applications; An outline of the IBM Advanced Process Control (APC) logic and system(s) that manage the focus correction sets; Long term, historical focus correction data for stable 45nm processes as well as development stage 32nm processes; Practical issues encountered and possible enhancements to the methodology.
As the advanced IC device process shrinks to below sub-micron dimensions (65nm, 45 nm and beyond), the overall CD error budget becomes more and more challenging. The impact of lithography process parameters other than exposure energy and defocus on final CD results cannot be ignored any more. In this paper we continue the development of an advanced control system, which can be used to detect, classify and correct up to 5 lithography parameters. Sets of focus exposure matrix (FEM) models are first set up with different DOE process conditions split. And photoresist profiles of specially designed scatterometry CD mark are then fitted to models (Neural Network Model or standard polynomial model). Based on these calibrated models, not only exposure and defocus but also PEB temperature, lens aberration, etc. can be estimated. This approach utilizes information of resist CD, height, sidewall and feature type dependent bias to classify different lithography parameters and therefore can give very accurate estimation of lithography parameters like energy, focus, PEB, spherical aberration and coma aberration. The new approach does not need phase shift mask or other specially designed mask, so it can be used by most of mass production Fabs and used for process monitoring and matching on inline production wafer.
The scatterometry or OCD (Optical CD) metrology technique has in recent years moved from being a general purpose CD metrology technique to one that addresses the metrology needs of process monitoring and control, where its strengths can be fully utilized. With the significant advancements that have been made in both hardware and software design, the setup time required to build complex models and solutions has been significantly reduced. Whilst the application of scatterometry to process control has clearly shown its merits, the question still arises as to how accurately the process corrections to feed forward or feedback for process control can be extracted? In this work we critically examine the accuracy of scatterometry with respect to process control by comparing three hardware platforms, on a simple litho stack. The impact of hardware design is discussed as well as the 'setup' of the modeled parameters on the final measurement result. It will be shown that informations extracted based on scatterometry measurements must be true to process variation and independent of the hardware design. Our results will show that the ability to use scatterometry effectively for process control ultimately lies in the ability to accurately determine the changes that have occurred in the process and to be able to extract appropriate process corrections for feedback or feed forward control; allowing these changes to be accurately corrected. To do this the metrology validation extends beyond the typical metrology metrics such as precision and TMU; metrology validation with respect to process control must encompass accurate determination of process corrections to ensure a process tool and/or process stays at the set point.
Lithography process control remains a significant challenge in modern semiconductor manufacturing. Metrology efforts must overcome the complexity of the lithography process, as well as the number of process elements that contribute to overall process yield. One specific area of concern is lithography tool focus control. It is vital to control photolithography tool focus during the photoresist development step with a high degree of precision and accuracy. Furthermore, dose variations can compound the difficulty in determining focus. The lenses used in photolithography tools have a very limited depth of focus, so utmost precision is necessary. Tools that are in focus will result in sharper and better controlled features, while tools that are out of focus will result in improperly developed photoresist features. Angular scatterometry is a technology well-suited for lithography inspection and process control because it provides rapid measurement data and can be used for the measurement of resist line profiles. We report on model-based methods for focus control and their application towards photolithography control in a production setting. Topics of discussion include the effect of model parameter selection for focus metrics on focus curve quality and accuracy, as well as the effect of grating target design on focus sensitivity and accuracy. Measurement data using this focus technique in a production setting will be presented.
Spectroscopic critical dimension (SCDTM) metrology on line gratings has previously been shown to be a sensitive and useful technique for monitoring lithographic focus and exposure conditions. Line end shortening (LES) effects are sensitive to focus and potentially more sensitive to focus variation than side wall angle or other profile parameters of line gratings. Rectangular line segment structures that exhibit line-end shortening behavior are arranged in a rectangular two-dimensional (2D) array to provide a scatterometry signal sensitive to the profile of the thousands of line ends in the measurement beam spot. Spectroscopic ellipsometry (SE)-based scatterometry measurements were carried out on 2D array targets of rectangular features exposed in a focus-exposure matrix (FEM). The focus and exposure sensitivities of multiple shape parameters were found to be good and uniquely separable. In addition, the side wall angle of the line ends was found to be nearly linearly dependent on focus and provide necessary focus direction information. Focus and exposure can be determined from SCD measurements by applying a model generated to describe the focus-exposure behavior of multiple shape parameters using KLA Tencor's KT Analyzer software. Several different models based on different combinations of shape parameters were evaluated. Focus measurement precision of 3nm 3sigma was obtained, which will be useful for lithography processes with tight depth of focus.
Numerous metrology tools, techniques and methods are used by the industry to setup and qualify exposure tools for production. Traditionally, different metrology techniques and tools have been used to setup dose, focus and overlay optimally and they do so independently. The methods used can be cumbersome, have the potential to interfere with each other and some even require an unacceptable amount of costly exposure tool time for data acquisition. In this work, we present a method that uses an advanced angle-resolved scatterometry metrology tool that has the capability to measure both CD and overlay. By using a technique to de-convolve dose and focus based on the profile measurement of a well characterized process monitor target, we show that the dose and focus signature of a high NA 193nm immersion scanner can be effectively measured and corrected. A similar approach was also taken to address overlay errors using the diffraction based overlay capability of our metrology tool. We demonstrate the advantage of having a single metrology tool solution, which enables us to reduce dose, focus and overlay signatures to a minimum.
As critical dimension (CD) control requirements increase and process windows decrease, it is now of even higher importance to be able to determine and separate the sources of CD error in an immersion cluster, in order to correct for them. It has already been reported that the CD error contributors can be attributed to two primary lithographic parameters: effective dose and focus. In this paper, we demonstrate a method to extract effective dose and focus, based on diffraction based optical metrology (scatterometry). A physical model is used to describe the CD variations of a target with controlled focus and dose offsets. This calibrated model enables the extraction of effective dose and focus fingerprints across wafer and across scanner exposure field. We will show how to optimize the target design and the process conditions, in order to achieve an accurate and precise de-convolution over a larger range of focus and dose than the expected variation of the cluster. This technique is implemented on an ASML XT:1900Gi scanner interfaced with a Sokudo RF3S track. The systematic focus and dose fingerprints obtained by this de-convolution technique enable identification of the specific contributions of the track, scanner and reticle. Finally, specific corrections are applied to compensate for these systematic CD variations and a significant improvement in CD uniformity is demonstrated.
Depth of Focus (DOF) and exposure latitude requirements have long been ambiguous. Techniques range from scaling values from previous generations to summing individual components from the scanner. Even more ambiguous is what critical dimension (CD) variation can be allowed to originate from dose and focus variation. In this paper we discuss a comprehensive approach to measuring focus variation that a process must be capable of handling. We also describe a detailed methodology to determine how much CD variation can come from dose and focus variation. This includes examples of the statistics used to combine individual components of CD, dose and focus variation.
To determine the magnitude of the exposure latitude required for a process to be manufacturable, additional factors are considered that have a similar relationship between linewidth variation and image log-slope. Such parameters include resist thickness, flare, post-exposure bake temperature, and line-edge roughness. To obtain consistency between theory and experiment it is necessary to use the resist-edge log-slope generalization of image log-slope. Inclusion of these additional factors increases the required exposure latitude five to six times more than would be considered necessary from exposure tool dose control alone.
This paper discusses the use of scatterometry for scanner focus control in hyper-NA lithography. A variety of techniques based on phase shift technology have been traditionally used to monitor scanner focus. Recently scatterometry has offered significant promise as an alternate technique to monitor both focus and dose. In this study, we make careful comparisons of a Scatterometry-based Focus-Dose Monitoring (SFDM) technique to Phase-grating Focus Monitoring (PGFM). We discuss the operating principles of these techniques and compare the sensitivity of SFDM to PGFM. In addition, the variation observed in characterizing intra-field and across-wafer behavior of a hyper-NA immersion scanner is described when using these different techniques.
The ever shrinking lithography process window requires us to maximize our process window and minimize tool-induced process variation, and also to quantify the disturbances to an imaging process caused upstream of the imaging step. Relevant factors include across-wafer and wafer-to-wafer film thickness variation, wafer flatness, wafer edge effects, and design-induced topography. We quantify these effects and their interactions, and present efforts to reduce their harm to the imaging process. We also present our effort to predict design-induced focus error hot spots at the edge of our process window. The collaborative effort is geared towards enabling a constructive discussion with our design team, thus allowing us to prevent or mitigate focus error hot spots upstream of the imaging process.
Advanced 193 nm lithographic processes will require defocus control for product wafers in order to meet CD and profile requirements in the future. Dose control is already required. The interaction of product wafer materials with lithography requires additional controls beyond tool monitoring. While scatterometry has demonstrated excellent ability to extract effective defocus and dose information from monitor wafers, the addition of product film stacks introduces several issues for this technique. The additional complexity of model generation and the sensitivity to under-layer thickness and optical property variation are among these. A CDSEM technique for lithography focus monitoring overcomes these issues provided it has sufficient precision and relative accuracy. In this paper, we report on comparative studies of two CDSEM techniques. One technique uses angled e-beam to better view the sidewall for edge width measurement. The angle of the beam from normal incidence is considerably larger than previously explored thereby enabling sensitive measurements on shallower structures. The other technique introduces new target designs particularly suited to CDSEM measurement that have enhanced sensitivity to focus and dose. Implementation of these techniques requires expanded sampling during the course of a single measurement in order to suppress roughness. The small target size of these structures enables applications with targets in product kerf and embedded within the circuit. In summary, these methods enable the measurement of dose and focus variations on product wafers.
We derive a physical model to describe the dependence of pattern dimensions on dose, defocus and blur. The coefficients of our model are constants of a given lithographic process. Model inversion applied to dimensional measurements then determines effective dose, defocus and blur for wafers patterned with the same process. In practice, our approach entails the measurement of proximate grating targets of differing dose and focus sensitivity. In our embodiment, the measured attribute of one target is exclusively sensitive to dose, whereas the measured attributes of a second target are distinctly sensitive to defocus and blur. On step-and-scan exposure tools, z-blur is varied in a controlled manner by adjusting the across slit tilt of the image plane. The effects of z-blur and x,y-blur are shown to be equivalent. Furthermore, the exposure slit width is shown to determine the tilt response of the grating attributes. Thus, the response of the measured attributes can be characterized by a conventional focus-exposure matrix (FEM), over which the exposure tool settings are intentionally changed. The model coefficients are determined by a fit to the measured FEM response. The model then fully defines the response for wafers processed under "fixed" dose, focus and blur conditions. Model inversion applied to measurements from the same targets on all such wafers enables the simultaneous determination of effective dose and focus/tilt (DaFT) at each measurement site.
Resolution enhancement techniques (RET), immersion lithography, and Design for Manufacturing (DFM) are all geared towards increasing the lithographic process window to enable the ever more difficult processing demands of semiconductor manufacturing. It is well understood that there is a trade-off between depth of focus (DOF) and exposure dose latitude (EL), as well as best focus (BF) and best exposure dose (BE), in which a Manufacturable Process Window (MPW) must be established and centered. Oftentimes it is overlooked that this balance needs to be maintained across multiple dimensions including spatial (e.g. across field), density (e.g. dense, iso), temporal, tool-to-tool, etc. To maintain this critical balance, both test wafers and product wafers need to be monitored and analyzed in order to support Advanced Process Control (APC) and Automated Equipment Control (AEC). In this work we establish a method to optimize process window by using an integrated analysis workstation based on measurements from both optical and e-beam metrology. By applying this method, we demonstrate a MPW on daily FEM and nominal wafers already used at IMEC for daily process qualification.
Optical properties (n&k) of the material films under measurement are commonly assumed invariant and fixed in scatterometry modeling. This assumption keeps the modeling simple by limiting the number of floating parameters in the model. Such scatterometry measurement has the potential to measure with high precision some of the profile parameters (CD, Sidewall angle). The question is: if the optical properties modeled as "fixed" are actually changing - would this modeling assumption impact the accuracy of reported geometrical parameters? Using the example of a resist profile measurement, we quantify the "bias" effect of un-modeled variation of optical properties on the accuracy of the reported geometry by utilizing a traditional fixed n&k model. With a second model we float an additional optical parameter and lower the bias of the reported values - at the expense of slightly increased "noise" of the measurement (more floating parameters - less precision). Finally, we extend our multi-stack approach (previously introduced as enabler to the product-driven materials characterization methodology) to augment the spectral information and increase both precision and accuracy through the simultaneous modeling of multiple targets
The process window for state of the art chip manufacturing continues to decrease, driven by higher NA exposure tools and lower k1 values. The benefits of immersion lithography for Depth of Focus (DoF) are well known. Yet even with this immersion boost, NA=1.35 tools can push DoF into sub-100nm territory. In addition, immersion processes are subject to new sources of dose and focus variation. In order to realize the full potential of immersion lithography, it is necessary to characterize, understand and attack all sources of process variation. Previous work has established our dose/focus metrology capability 1 , in which we expose Process Monitor Grating (PMG) targets with high sensitivity to focus, measure the PMGs using scatterometry, and use the Ausschnitt dose/focus deconvolution approach to determine focus errors to within a few nm and dose errors to within 0.1%. In this paper, we concentrate on applying this capability to the detailed measurements of immersion photoclusters utilizing ASML exposure tools. Results will include: • comparison of Twinscan 1700i and 1900i focus capability • effectiveness of the Reticle Shape Correction (RSC) for non-flat reticles • visualization of non-flat wafer chucks, tilted image planes, and other systematic focus error components • tracking of tool trends over time, using automated monitor wafer flows The highly systematic nature of the observed focus errors suggest potential for future improvements in focus capability.
We have developed in-line dose and focus monitoring techniques for the detailed analysis of critical dimension error and accurate process control. From exposed wafers, effective does and focus are measured with specificed monitor marks built on a reticle. The contributions of effective dose and focus to critical dimension error on device chips were clarified in a fabrication proces of 110 nm isolated pattern with a KrF scanner. The critical dimensions error was described as a function of effective dose and focus, which include various process fluctuations. We could determine whether current exposure settings such as dose input and focus input were adequate or not. Based on the experimental data, we estimated the benefit of simultaneous Run-to-Run control of dose and focus. The estimation clarifies that it realizes total critical dimension control including Run-to-Run and intra-Run.
A new lithographic test pattern, the focus monitor, is introduced. Through the use of phase shift techniques, focus errors translate into easily measurable overlay shifts in the printed pattern. Each individual focus monitor pattern can be directly read for the sign and magnitude of the focus error. This paper presents a detailed verification of the validity of this approach, along with several preliminary applications.
The paper introduces an improved, physics-based function for fitting lithographic data from focus-exposure matrices. Unlike simple polynomial functions, the coefficients of this equation offer physical insight into the meaning and nature of the data. Derivation of this equation from first principles of the physics of lithographic imaging is presented. Examples based on typical experimental data are shown and the advantages of using a physics-based fitting function is described based on improved fitting and noise filtering.
This paper is the third of a series that defines a new approach to in-line lithography control. The first paper described the use of optically measurable line-shortening targets to enhance signal-to-noise and reduce measurement time. The second described the dual-tone optical critical dimension (OCD) measurement and analysis necessary to distinguish dose and defocus. Here we describe the marriage of dual-tone OCD to SEM-CD metrology that comprises what we call 'process window metrology' (PWM), the means to locate each measured site in dose and focus space relative to the allowed process window. PWM provides in-line process tracking and control essential to the successful implementation of low-k lithography.
A novel exposure monitor structure (EMS) is proposed as a sensitive
means to monitor or measure effective exposure dose in the optical
lithographic printing. Like windows of graduated transmittance, it is
very sensitive to dose, but not to focus. On the other hand, EMS is
compact to allow placement in the kerf or device layouts and it is
measurable by automated rctetrology tools.
EMS is built in the conventional reticles by arranging a set of
stripes of chrome and glass on the period near MTF cut-off in a manner
simulating transmission wedges on both sides of a conventional line.
Since the fine structure of EMS is not resolved by the optical lithography
tool used for printing, it performs like a transmission wedge.
Conventional linewidth measurements of a printed EMS image are used to
monitor exposure dose. Coupled with a calibration curve, sensitive
dose measurements can also be made.
Modelling of printing of EMS and of a conventional line on an optical
stepper indicates that printed linewidth of EMS changes much faster as
the function of dose. EMS also displays relatively little sensitivity
to defocus.
Conventional 5X reticles with EMS were built. Exposure sequences were
carried out on a G-line stepper. Linewidth measurements of printed
EMS and of conventional lines were done on a typical linewidth measurement
tool. Experiment confirms that linewidth of printed EMS is
much more sensitive to dose than that of a conventional line.
Once linewidth of EMS is established at the desirable dose, EMS may be
used to set that dose in the subsequent runs. With more accurate dose
set-up and job disposition decisions, higher product tolerance can be
achieved with existing equipment and process.
We address here the optimization of the photolithographic process for submicron design rules. We describe a new method to determine the exposure-focus window corresponding to a user-specified design rule. The change in size and position of the window with changes in design rule, and in process and equipment variables, is the basis for optimization. We describe the implementation of our optimization approach in the MONO-LITH 11/88 Workstation; a computer aided engineering tool that standardizes data acquisition, analysis and presentation, independent of metrology or sampling approach.
A focus monitor technology for attenuated PSM under annular illumination has been developed as an in-line quality control. The focus monitor pattern on a reticle employs a pair of grouped lozenge-shaped opening patterns in attenuated phase shifting region. Since the phase shifting angles of the light passing through the first and second opening patterns are 90 degrees and 180 degrees, respectively, the best focus position for the first pattern shifts to that for the second pattern. The subtraction of the length of the patterns is a linear function of the actual focal position printed on the wafer. Therefore, the effective focal position can be extracted by measuring the subtraction of the measured length. A high resolution of 10-nm defocus could be achieved by this technique.
We describe an approach to pattern metrology that enables the simultaneous determination of critical dimensions, overlay and film thickness. A single optical system captures nonzero- and zero-order diffracted signals from illuminated grating targets, as well as unpatterned regions of the surrounding substrate. Differential targets provide in situ dimensional calibration. CD target signals are analyzed to determine average dimension, profile attributes, and effective dose and defocus. In turn, effective dose and defocus determines all CDs pre-correlated to the dose and focus settings of the exposure tool. Overlay target signals are analyzed to determine the relative reflectivity of the layer pair and the overlay error between them. Compared to commercially available pattern metrology (SEM, optical microscopy, AFM, scatterometry and schnitzlometry), our approach promises improved signal-to-noise, higher throughput and smaller targets. We have dubbed this optical chimera MOXIE (Metrology Of eXtremely Irrational Exuberance).
Manufacturing control of a lithographic process must guarantee that the pattern features on a masking level stay within a common process window, the focus-exposure space over which all pattern tolerances are met. To do so at focus latitudes below 1 micrometers , simultaneous determination and correction of dose and defocus error is required. In-line metrology practice has been to measure a single pattern attribute, usually the dimension of the smallest feature, at each of several locations on a wafer. Since the measurement of one pattern attribute, regardless of its accuracy or precision, cannot distinguish two variables - this approach is inherently inadequate for lithography control. We demonstrate how dose and defocus can be derived from the attributes of dual-tone, optically measurable targets on product wafers. Our method is applied to the in-line control of sub-0.25 micrometers step-and-scan lithography.