Advanced technology nodes, 10nm and beyond, employing multi-patterning techniques for pitch reduction pose new process and metrology challenges in maintaining consistent positioning of structural features. Self-Aligned Quadruple Patterning (SAQP) process is used to create the Fins in FinFET devices with pitch values well below optical lithography limits. The SAQP process bares compounding effects from successive Reactive Ion Etch (RIE) and spacer depositions. These processes induce a shift in the pitch value from one fin compared to another neighboring fin. This is known as pitch walking. Pitch walking affects device performance as well as later processes which work on an assumption that there is consistent spacing between fins. In SAQP there are 3 pitch walking parameters of interest, each linked to specific process steps in the flow. These pitch walking parameters are difficult to discriminate at a specific process step by singular evaluation technique or even with reference metrology such as Transmission Electron Microscopy (TEM). In this paper we will utilize a virtual reference to generate a scatterometry model to measure pitch walk for SAQP process flow.
Complexity of process steps integration and material systems for next-generation technology nodes is reaching unprecedented levels, the appetite for higher sampling rates is on the rise, while the process window continues to shrink. Current thickness metrology specifications reach as low as 0.1A for total error budget – breathing new life into an old paradigm with lower visibility for past few metrology nodes: accuracy. Furthermore, for advance nodes there is growing demand to measure film thickness and composition on devices/product instead of surrogate planar simpler pads. Here we extend our earlier work in Hybrid Metrology to the combination of X-Ray based reference technologies (high performance) with optical high volume manufacturing (HVM) workhorse metrology (high throughput). Our stated goal is: put more “eyes” on the wafer (higher sampling) and enable move to films on pattern structure (control what matters). Examples of 1X front-end applications are used to setup and validate the benefits.
Copper interconnects have been adopted in advanced semiconductor manufacturing due to benefits of reduced RC delay, cross talk and power consumption. With each technology node, interconnects reduce in size resulting in increased line resistivity, a critical metric in determining the device performance. Reactive Ion Etching (RIE) and Copper Chemical Mechanical Polishing (Cu CMP) are two of the key back end of the line (BEOL) processes that affect the interconnect performance. Due to variations from incoming processes and the inherent variability induced by these processes, dielectric trench depth and resulting copper line height variations that can potentially result from these processes have direct impact to RC delay.
Traditional inline metrology methods used are time consuming and do not provide the needed wafer level metrics. In addition, measurement of remaining dielectric thickness on solid pads is not a good representative of the actual device structures and has been inaccurate for process due to dishing of the copper pads. Efficient control of BEOL processes requires measurement of metal line thickness and other critical profile parameters from which resistance can be extracted. In order to relate BEOL process steps and understand their interactions, it is necessary to have a directly comparable measurement methodology on a similar measurement structure.
Over the past several years, scatterometry has been proven as the only metrology method to provide the full profile information of the Cu lines. Scatterometry is a diffraction based optical measurement technique using Rigorous Coupled Wave Analysis (RCWA), where light diffracted from a periodic structure is used to characterize the details of profile. Unique algorithms, such as Holistic Metrology can be used to make the scatterometry development process faster.
In this paper, we will present how scatterometry can be used to measure copper line height on 3D structures and how feed forward from RIE can be applied for control of Cu CMP process for 20nm technology node. The importance of incoming trench depth variations is demonstrated for CMP polish time control in order to stabilize the copper line height. Validation data is presented for different scatterometry models including accuracy, repeatability and DoE tracking. Electrical resistance is shown to correlate to the copper trench profile measured by scatterometry. The paper will demonstrate the capability for reducing copper line height variation and the correlation of the reducing trench height variation to improved stabilization of electrical resistance.
CD and shape control of extreme ultraviolet lithography (EUVL) structures is critical to ensure patterning performance at the 10 nm technology node and beyond. The optimum focus/dose control by EUV scanner is critical for CD uniformity, and the scanner depends on reliable and rapid metrology feedback to maintain control. The latest advances in scatterometry such as ellipsometry (SE), reflectometry (NISR), and Mueller matrix (MM) offers complete pattern profile, critical dimensions (CD), side-wall angles, and dimensional characterization. In this study, we will present the evaluation results of CD uniformity and focus dose sensitivity of line and space EUV structures at the limit of current ASML NXE 3100 scanner printability and complex 3D EUV structures. The results will include static and dynamic precision and CD-SEM correlation data.
Optical critical dimension (OCD) metrology using scatterometry has been demonstrated to be a viable solution for fast and non-destructive in-line process control and monitoring. As extreme ultraviolet lithography (EUVL) is more widely adopted to fabricate smaller and smaller patterns for electronic devices, scatterometry faces new challenges due to several reasons. For 14nm node and beyond, the feature size is nearly an order of magnitude smaller than the shortest wavelength used in scatterometry. In addition, thinner resist layer is used in EUVL compared with conventional lithography, which leads to reduced measurement sensitivity. Despite these difficulties, tolerance has reduced for smaller feature size. In this work we evaluate 3D capability of scatterometry for EUV process using spectroscopic ellipsometry (SE). Three types of structures, contact holes, tip-to-tip, and tip-to-edge, are studied to test CD and end-gap metrology capabilities. The wafer is processed with focus and exposure matrix. Good correlations to CD-SEM results are achieved and good dynamic precision is obtained for all the key parameters. In addition, the fit to process provides an independent method to evaluate data quality from different metrology tools such as OCD and CDSEM. We demonstrate 3D capabilities of scatterometry OCD metrology for EUVL using spectroscopic ellipsometry, which provides valuable in-line metrology for CD and end-gap control in electronic circuit fabrications.
Diffraction-based overlay (DBO) technologies have been developed to address the overlay metrology
challenges for 22nm technology node and beyond. Most DBO technologies require specially designed targets that
consist of multiple measurement pads, which consume too much space and increase measurement time. The traditional
empirical approach (eDBO) using normal incidence spectroscopic reflectometry (NISR) relies on linear response of the
reflectance with respect to overlay displacement within a small range. It offers convenience of quick recipe setup since
there is no need to establish a model. However it requires three or four pads per direction (x or y) which adds burden to
throughput and target size. Recent advances in modeling capability and computation power enabled mDBO, which
allows overlay measurement with reduced number of pads, thus reducing measurement time and DBO target space. In
this paper we evaluate the performance of single pad mDBO measurements using two 3D targets that have different
grating shapes: squares in boxes and L-shapes in boxes. Good overlay sensitivities are observed for both targets. The
correlation to programmed shifts and image-based overlay (IBO) is excellent. Despite the difference in shapes, the
mDBO results are comparable for square and L-shape targets. The impact of process variations on overlay measurements
is studied using a focus and exposure matrix (FEM) wafer. Although the FEM wafer has larger process variations, the
correlation of mDBO results with IBO measurements is as good as the normal process wafer. We demonstrate the
feasibility of single pad DBO measurements with faster throughput and smaller target size, which is particularly
important in high volume manufacturing environment.
Resolution enhancement techniques such as double patterning (DP) processes are implemented to achieve
lower critical dimension (CD) control tolerances. However the design complications, overlay resulting
from multiple exposures, and production cost limit the DP usage. EUVL offers the most promising
patterning technology to be adopted for 14nm and beyond due to simplicity and cost advantage estimates.
However, EUVL is also prone to number of patterning challenges that are unique to EUV, such as
orientation dependent pattern placement errors resulting from mask shadowing effect, flare(leads to CD
non-uniformity) and non-flatness (leads to overlay errors). Even though the shadowing effects can be
corrected by means of OPC and mask stack design, there is a need to monitor the systemic errors due to HV
bias in order to control the lithographic process. In this paper, we will report the measurement sensitivity
of EUVL targets (CD, height and sidewall angle), systemic CD errors (H-V bias) and feedback for OPC
correction by scatterometry. We will also report the measurement precision, accuracy and matching for
Spacer defined double patterning processes consists of multiple deposition, post strips and etch steps and is
inherently susceptible to the cumulative effects of defects from each process step leading to higher rate of
defect detection. CD distortions and CD non-uniformity leads to DPT overlay errors. This demands
improved critical dimension uniformity (CDU) and overlay control. Scatterometry technique enables the
characterization and control the CD uniformity and provision to monitor stepper and scanner characteristics
such as focus and dose control. While CDSEM is capable of characterizing CD and sidewall angle, is not
adequate to resolve shape variations, such as footing and top rounding and spacers with leaning angles,
during the intermediate process steps. We will characterize direct low temperature oxide deposition on
resist spacer with fewer core films and reduced number of processing and metrology control steps.
Metrology characterization of SADP and resist core transferred spacers at various process steps will be
performed by scatterometry using spectroscopic ellipsometry and reflectometry. We will present CD
distribution (CDU) and profile characterization for core formation, spacer deposition and etch by advanced
optical scatterometry and also validate against CDSEM.
As the dimensions of integrated circuit continue to shrink, diffraction based overlay (DBO) technologies have
been developed to address the tighter overlay control challenges. Previously data of high accuracy and high precision
were reported for litho-etch-litho-etch double patterning (DP) process using normal incidence spectroscopic
reflectometry on specially designed targets composed of 1D gratings in x and y directions. Two measurement methods,
empirical algorithm (eDBO) using four pads per direction (2x4 target) and modeling based algorithm (mDBO) using two
pads per direction (2x2 target) were performed. In this work, we apply DBO techniques to measure overlay errors for a
different DP process, litho-freeze-litho-etch process. We explore the possibility of further reducing number of pads in a
DBO target using mDBO. For standard targets composed of 1D gratings, we reported results for eDBO 2x4 targets,
mDBO 2x2 targets, and mDBO 2x1 target. The results of all three types of targets are comparable in terms of accuracy,
dynamic precision, and TIS. TMU (not including tool matching) is less than 0.1nm. In addition, we investigated the
possibility of measuring overlay with one single pad that contains 2D gratings. We achieved good correlation to blossom
measurements. TMU (not including tool matching) is ~ 0.2nm. To our best knowledge, this is the first time that DBO
results are reported on a single pad. eDBO allows quick recipe setup but takes more space and measurement time.
Although mDBO needs details of optical properties and modeling, it offers smaller total target size and much faster
throughput, which is important in high volume manufacturing environment.
Double patterning technology overlay errors result in critical dimension (CD) distortions, and CD nonuniformity leads to overlay errors, demanding increased critical dimension uniformity (CDU) and improved overlay control. Scatterometry techniques are used to characterize the CD uniformity, focus, and dose control. We present CDU and profile characterization for spacer double patterning structures by advanced scatterometry methods. Our results include normal incidence spectroscopic reflectometry (NISR) and spectroscopic ellipsometry (SE) characterization of CDU sensitivity in spacer double patterning stacks. We further show the results of spacer DP structures by NISR and SE measurements. Metrology comparisons at various process steps including litho, etch, and spacer, and validation of CDU and profile, are all benchmarked against traditional critical dimension scanning electron microscope measurements.
DPT overlay errors result in CD distortions and CD non-uniformity leads to overlay errors demanding
increased critical dimension uniformity (CDU) and improved overlay control. Scatterometry techniques are
used to characterize the CD uniformity, focus and dose control. We will present CD distribution (CDU) and
profile characterization for spacer double patterning structures by advanced scatterometry methods. Our
result will include NISR, and spectroscopic ellipsometry (SE) characterization of CDU sensitivity in spacer
double patterning stack. We will further show the results of spacer DP structures by NISR and SE
measurements. Metrology comparison at various process steps including litho, etch and spacer and
validation of CDU and profile; all benchmarked against traditional CDSEM measurements.
Diffraction based overlay (DBO) technologies have been developed to address the tighter overlay control
challenges as the dimensions of integrated circuit continue to shrink. Several studies published recently have
demonstrated that the performance of DBO technologies has the potential to meet the overlay metrology budget for
22nm technology node. However, several hurdles must be cleared before DBO can be used in production. One of the
major hurdles is that most DBO technologies require specially designed targets that consist of multiple measurement
pads, which consume too much space and increase measurement time. A more advanced spectroscopic ellipsometry (SE)
technology-Mueller Matrix SE (MM-SE) is developed to address the challenge. We use a double patterning sample to
demonstrate the potential of MM-SE as a DBO candidate. Sample matrix (the matrix that describes the effects of the
sample on the incident optical beam) obtained from MM-SE contains up to 16 elements. We show that the Mueller
elements from the off-diagonal 2x2 blocks respond to overlay linearly and are zero when overlay errors are absent. This
superior property enables empirical DBO (eDBO) using two pads per direction. Furthermore, the rich information in
Mueller matrix and its direct response to overlay make it feasible to extract overlay errors from only one pad per
direction using modeling approach (mDBO). We here present the Mueller overlay results using both eDBO and mDBO and compare the results with image-based overlay (IBO) and CD-SEM results. We also report the tool induced shifts (TIS) and dynamic repeatability.
The extension of optical lithography to 22nm and beyond by Double Patterning Technology is often challenged by CDU
and overlay control. With reduced overlay measurement error budgets in the sub-nm range, relying on traditional Total
Measurement Uncertainty (TMU) estimates alone is no longer sufficient. In this paper we will report scatterometry
overlay measurements data from a set of twelve test wafers, using four different target designs. The TMU of these
measurements is under 0.4nm, within the process control requirements for the 22nm node. Comparing the measurement differences between DBO targets (using empirical and model based analysis) and with image-based overlay data indicates the presence of systematic and random measurement errors that exceeds the TMU estimate.
The extension of optical lithography to 32nm and beyond is made possible by Double Patterning Techniques
(DPT) at critical levels of the process flow. The ease of DPT implementation is hindered by increased significance of
critical dimension uniformity and overlay errors. Diffraction-based overlay (DBO) has shown to be an effective
metrology solution for accurate determination of the overlay errors associated with double patterning [1, 2] processes. In
this paper we will report its use in litho-freeze-litho-etch (LFLE) and spacer double patterning technology (SDPT),
which are pitch splitting solutions that reduce the significance of overlay errors. Since the control of overlay between
various mask/level combinations is critical for fabrication, precise and accurate assessment of errors by advanced
metrology techniques such as spectroscopic diffraction based overlay (DBO) and traditional image-based overlay (IBO)
using advanced target designs will be reported. A comparison between DBO, IBO and CD-SEM measurements will be
reported. . A discussion of TMU requirements for 32nm technology and TMU performance data of LFLE and SDPT
targets by different overlay approaches will be presented.
As optical lithography advances to 32 nm technology node and beyond, double patterning technology (DPT)
has emerged as an attractive solution to circumvent the fundamental optical limitations. DPT poses unique demands on
critical dimension (CD) uniformity and overlay control, making the tolerance decrease much faster than the rate at which
critical dimension shrinks. This, in turn, makes metrology even more challenging. In the past, multi-pad diffractionbased
overlay (DBO) using empirical approach has been shown to be an effective approach to measure overlay error
associated with double patterning . In this method, registration errors for double patterning were extracted from
specially designed diffraction targets (three or four pads for each direction); CD variation is assumed negligible within
each group of adjacent pads and not addressed in the measurement. In another paper, encouraging results were reported
with a first attempt at simultaneously extracting overlay and CD parameters using scatterometry .
In this work, we apply scatterometry with a rigorous coupled wave analysis (RCWA) approach to characterize
two double-patterning processes: litho-etch-litho-etch (LELE) and litho-freeze-litho-etch (LFLE). The advantage of
performing rigorous modeling is to reduce the number of pads within each measurement target, thus reducing space
requirement and improving throughput, and simultaneously extract CD and overlay information. This method measures
overlay errors and CDs by fitting the optical signals with spectra calculated from a model of the targets. Good
correlation is obtained between the results from this method and that of several reference techniques, including empirical
multi-pad DBO, CD-SEM, and IBO. We also perform total measurement uncertainty (TMU) analysis to evaluate the
overall performance. We demonstrate that scatterometry provides a promising solution to meet the challenging overlay
metrology requirement in DPT.
Applications that require overlay measurement between layers separated by absorbing interlayer films (such as α-
carbon) pose significant challenges for sub-50nm processes. In this paper scatterometry methods are investigated as an
alternative to meet these stringent overlay metrology requirements. In this article, a spectroscopic Diffraction Based
Overlay (DBO) measurement technique is used where registration errors are extracted from specially designed
diffraction targets. DBO measurements are performed on detailed set of wafers with varying α-carbon (ACL)
thicknesses. The correlation in overlay values between wafers with varying ACL thicknesses will be discussed. The total
measurement uncertainty (TMU) requirements for these layers are discussed and the DBO TMU results from sub-50nm
samples are reviewed.
Demanding sub-45 nm node lithographic methodologies such as double patterning (DPT) pose significant challenges for
overlay metrology. In this paper, we investigate scatterometry methods as an alternative approach to meet these stringent
new metrology requirements. We used a spectroscopic diffraction-based overlay (DBO) measurement technique in
which registration errors are extracted from specially designed diffraction targets for double patterning. The results of
overlay measurements are compared to traditional bar-in-bar targets. A comparison between DBO measurements and
CD-SEM measurements is done to show the correlation between the two approaches. We discuss the total measurement
uncertainty (TMU) requirements for sub-45 nm nodes and compare TMU from the different overlay approaches.
As overlay budgets continue to shrink, there is an increasing need to more fully characterize the tools used
to measure overlay. In a previous paper, it was shown how a single-layer Blossom overlay target could be
utilized to measure aberrations across the field of view of an overlay tool in an efficient and low-cost
manner. In this paper, we build upon this method, and discuss the results obtained, and experiences gained
in applying this method to a fleet of currently operational overlay tools.
In particular, the post-processing of the raw calibration data is discussed in detail, and a number of different
approaches are considered. The quadrant-based and full-field based methods described previously are
compared, along with a half-field method. In each case we examine a number of features, including the
trade off between ease of use (including the total number of measurements required) versus sensitivity /
potential signal to noise ratio. We also examine how some techniques are desensitized to specific types of
tool or mark aberration, and suggest how to combine these with non-desensitized methods to quickly
identify these anomalies.
There are two distinct applications of these tool calibration methods. Firstly, they can be used as part of the
tool build and qualification process, to provide absolute metrics of imaging quality. Secondly, they can be
of significant assistance in diagnosing tool or metrology issues or providing preventative maintenance
diagnostics, as (as shown previously) under normal operation the results show very high consistency, even
compared to aggressive overlay requirements.
Previous work assumed that the errors in calibration, from reticle creation through to the metrology itself,
would be Gaussian in nature; in this paper we challenge that assumption, and examine a specific scenario
that would lead to very non-Gaussian behavior. In the tool build / qualification application, most scenarios
lead to a systematic trend being superimposed over Gaussian-distributed measurements; these cases are
relatively simple to treat. However, in the tool diagnosis application, typical behavior will be very non-
Gaussian in nature, for example individual outlier measurements, or exhibiting bimodal or other probability
In such cases, we examine the effect that this has on the analysis, and show that such anomalous behaviors
can occur "under the radar" of analyses that assume Gaussian behavior. Perhaps more interestingly, the
detection / identification of non-Gaussian behavior (as opposed to the parameters of a best fit Gaussian
probability density function) can be a useful tool in quickly isolating specific metrology problems. We also
show that deviation of a single tool, relative to the tool fleet, is a more sensitive indicator of potential
The extension of optical lithography to 32nm and beyond is dependent on double-patterning (DP) at critical levels. DP integration strategies result in added degrees of freedom for overlay variation. In particular, overlay control requires assessment of error among various mask/level combinations. The Blossom overlay metrology approach minimizes the size of the overlay marks associated with each mask/level while maximizing the density of marks within the overlay metrology tool's field of view (FOV). We examine Blossom enabled use cases in DP lithography control; specifically, within-field and multiple mask/level sampling.
Improved overlay capability and sampling to control advanced lithography has accelerated the need for compact, multilayer/
mask/field/mark overlay metrology. The Blossom approach minimizes the size of the overlay marks associated
with each layer while maximizing the density of marks within the overlay metrology tool's field of view (FOV). Here
we describe our progress implementing this approach in 45nm manufacturing.
In a previous publication, we introduced Blossom, a multi-layer overlay mark (Ausschnitt, et al. 2006, ).
Through further testing carried out since that publication, Blossom has been shown to meet the requirements
on current design rules (Ausschnitt, et al. 2007, ), while giving some unique benefits. However, as future
design rules shrink, efforts must be made now to ensure the extensibility of the Blossom technology.
Previous work has shown that the precision component of Total Measurement Uncertainty (TMU) can be
reduced by using extra redundancy in the target design, to achieve performance beyond that of a conventional
box-in-box measurement. However, improvements that single contributor to TMU would not be sufficient for
future design rules; therefore we have also to consider the Tool Induced Shift (TIS) variability and tool to
tool matching contributions to TMU.
In this paper, we introduce a calibration artifact, based on the Blossom technology. The calibration artifact is
both compact, and produced by standard lithography process, so it can be placed in a production scribe line if
required, reducing the need for special sets of calibration wafers compared to other possible calibration
methodologies. Calibration is currently with respect to the exposure tool / process / mask, which is arguably
more pertinent to good yield, and less expensive, than calibration to an external standard; externally
calibrated artifacts would be straightforward to manufacture if needed.
By using this artifact, we can map out remaining optical distortions within an overlay tool, to a precision
significantly better than the operational tool precision, in a way that directly relates to overlay performance.
The effect of process-induced mark uncertainties on calibration can be reduced by performing measurements
on a large number of targets; by taking multiple measurements of each target we can also use the artifact to
evaluate the current levels of process induced mark uncertainty. The former result leads to an improvement
method for TIS and matching capability. We describe the artifact and its usage, and present results from a
group of operational overlay tools.
We show how the use of this information also provides further insight into the layout optimizations discussed
previously (Binns et al. 2006 ). It provides the current limits of measurement precision and mark fidelity
with respect to target redundancy, enabling us to use a predictive cost-benefit term in the optimization.
Finally, examining the bulk behaviour of a fleet of overlay tools, allows us to examine how future mark
layouts can also contribute to minimizing TMU rather than just precision.
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.
The deployment of angular scatterometry as a powerful and effective process control methodology has recently included the measurement of etched metal features in a typical complex Aluminum stack. With the control of metal process steps taking a more critical role in silicon manufacturing, a fast, reproducible and accurate methodology for measuring CD and depth is necessary. With the half-pitch of the metal pattern being as low as the minimum device feature, etch rate measurements on above-micron test structures are hardly indicative of the pattern-dependent etch profiles and behavior. Angular scatterometry offers a non-destructive, fast and powerful approach for measuring the profiles of the yield-relevant array features in metal applications.
In this work we demonstrate the application of angular scatterometry to the qualification of metal etchers. Etch depth is difficult to control and must be inspected with slow techniques such as profilometry. In addition to the slow response time and sparse radial sampling, contact profilometry is susceptible to residual resist and polymer residue as well as to the variations in the TiN ARC layer affecting the measurement of the Aluminum etch rates. We show that the choice of a suitable profile model and accurate knowledge of the optical properties allow scatterometry to overcome all of these traditional challenges.
We demonstrate that angular scatterometry is sensitive to the parameters of interest for controlling metal etchers, specifically etch depth, CD and profile. Across an experimental design that introduced intentional variations in these parameters, angular scatterometry results were able to track the variations accurately. In addition, profile results determined through scatterometry compare favorably with cross-sectional SEM images and measurements. Measurement precision results will also be presented.