Scanner Focus window of the lithographic process becomes much smaller due to the shrink of the device node and multipatterning approach. Consequently, the required performance of scanner focus becomes tighter and more complicated. Focus control/monitoring methods such as “field-by-field focus control” or “intra-field focus control” is a necessity. Moreover, tight scanner focus performance requirement starts to raise another fundamental question: accuracy of the reported scanner focus. <p> </p>The insufficient accuracy of the reported scanner focus using the existing methods originates from: <p> </p>a) Focus measurement quality, which is due to low sensitivity of measured targets, especially around the nominal production focus. <p> </p>b) The scanner focus is estimated using special targets, e.g. large pitch target and not using the device-like structures (irremovable aberration impact). <p> </p>Both of these factors are eliminated using KLA-Tencor proprietary “Focus Offset” technology.
As overlay budget continues to shrink, an improved analysis of the different contributors to this budget is needed. A
major contributor that has never been quantified is the accuracy of the measurements. KLA-Tencor developed a quality
metric, that calculates and attaches an accuracy value to each OVL target. This operation is performed on the fly during
measurement and can be applied without affecting MAM time or throughput. Using a linearity array we demonstrate that
the quality metric identifies targets deviating from the intended OVL value, with no false alarms.
Currently, the performance of overlay metrology is evaluated mainly based on random error contributions such as
precision and TIS variability. With the expected shrinkage of the overlay metrology budget to < 0.5nm, it becomes
crucial to include also systematic error contributions which affect the accuracy of the metrology. Here we discuss
fundamental aspects of overlay accuracy and a methodology to improve accuracy significantly.
We identify overlay mark imperfections and their interaction with the metrology technology, as the main source of
overlay inaccuracy. The most important type of mark imperfection is mark asymmetry. Overlay mark asymmetry leads
to a geometrical ambiguity in the definition of overlay, which can be ~1nm or less. It is shown theoretically and in
simulations that the metrology may enhance the effect of overlay mark asymmetry significantly and lead to metrology
inaccuracy ~10nm, much larger than the geometrical ambiguity. The analysis is carried out for two different overlay
metrology technologies: Imaging overlay and DBO (1st order diffraction based overlay). It is demonstrated that the
sensitivity of DBO to overlay mark asymmetry is larger than the sensitivity of imaging overlay.
Finally, we show that a recently developed measurement quality metric serves as a valuable tool for improving overlay
metrology accuracy. Simulation results demonstrate that the accuracy of imaging overlay can be improved significantly
by recipe setup optimized using the quality metric. We conclude that imaging overlay metrology, complemented by
appropriate use of measurement quality metric, results in optimal overlay accuracy.
We have developed a new scheme of process control combining a CD metrology system and an exposure tool. A new
model based on Neural Networks has been created in KLA-Tencor's "KT Analyzer" which calculates the dose and
focus errors simultaneously from CD parameters, such as mid CD and height information, measured by a scatterometry
(OCD) measurement tool. The accuracy of this new model was confirmed by experiment. Nikon's "CDU master" then
calculated the control parameters for dose and focus per each field from the dose and focus error data of a reference
wafer provided by KT Analyzer. Using the corrected parameters for dose and focus from CDU master, we exposed
wafers on an NSR-S610C (ArF immersion scanner), and measured the CDU on a KLA SCD100 (OCD tool). As a result,
we confirmed that CDU in the entire wafer can be improved more than 60% (from 3.36nm (3σ) to 1.28nm (3σ)).
The double patterning (DPT) process is foreseen by the industry to be the main solution for the 32 nm technology node
and even beyond. Meanwhile process compatibility has to be maintained and the performance of overlay metrology has
to improve. To achieve this for Image Based Overlay (IBO), usually the optics of overlay tools are improved. It was also
demonstrated that these requirements are achievable with a Diffraction Based Overlay (DBO) technique named SCOL<sup>TM</sup>
. In addition, we believe that overlay measurements with respect to a reference grid are required to achieve the
required overlay control . This induces at least a three-fold increase in the number of measurements (2 for double
patterned layers to the reference grid and 1 between the double patterned layers). The requirements of process
compatibility, enhanced performance and large number of measurements make the choice of overlay metrology for DPT
In this work we use different flavors of the standard overlay metrology technique (IBO) as well as the new technique
(SCOL) to address these three requirements. The compatibility of the corresponding overlay targets with double
patterning processes (Litho-Etch-Litho-Etch (LELE); Litho-Freeze-Litho-Etch (LFLE), Spacer defined) is tested. The
process impact on different target types is discussed (CD bias LELE, Contrast for LFLE). We compare the standard
imaging overlay metrology with non-standard imaging techniques dedicated to double patterning processes (multilayer
imaging targets allowing one overlay target instead of three, very small imaging targets). In addition to standard designs
already discussed , we investigate SCOL target designs specific to double patterning processes. The feedback to the
scanner is determined using the different techniques. The final overlay results obtained are compared accordingly. We
conclude with the pros and cons of each technique and suggest the optimal metrology strategy for overlay control in
double patterning processes.
The overlay metrology budget is typically 1/10 of the overlay control budget resulting in overlay metrology total
measurement uncertainty requirements of 0.57 nm for the most challenging use cases of the 32nm technology generation.
Theoretical considerations show that overlay technology based on differential signal scatterometry (SCOL<sup>TM</sup>) has
inherent advantages, which will allow it to achieve the 32nm technology generation requirements and go beyond it.
In this work we present results of an experimental and theoretical study of SCOL. We present experimental results,
comparing this technology with the standard imaging overlay metrology. In particular, we present performance results,
such as precision and tool induced shift, for different target designs. The response to a large range of induced
misalignment is also shown. SCOL performance on these targets for a real stack is reported. We also show results of
simulations of the expected accuracy and performance associated with a variety of scatterometry overlay target designs.
The simulations were carried out on several stacks including FEOL and BEOL materials. The inherent limitations and
possible improvements of the SCOL technology are discussed. We show that with the appropriate target design and
algorithms, scatterometry overlay achieves the accuracy required for future technology generations.
Resolution enhancement in advanced optical lithography will reach a new plateau of complexity at the 32 nm design rule
manufacturing node. In order to circumvent the fundamental optical resolution limitations, ultra low k<sub>1</sub> printing
processes are being adopted, which typically involve multiple exposure steps. Since alignment performance is not
fundamentally limited by resolution, it is expected to yield a greater contribution to the effort to tighten lithographic error
budgets. In the worst case, the positioning budget usually allocated to a single patterning step is divided between two. A
concurrent emerging reality is that of high order overlay modeling and control. In tandem with multiple exposures, this
trend creates great pressure to reduce scribeline target real estate per exposure. As the industry migrates away from
metrology targets formed from large isolated features, the adoption of dense periodic array proxies brings improved
process compatibility and information density as epitomized by the AIM target<sup>1</sup>. These periodic structures enable a
whole range of new metrology sensor architectures, both imaging and scatterometry based, that rely on the principle of
diffraction order control and which are no longer aberration limited. Advanced imaging techniques remain compatible
with side-by-side targets while scatterometry methods require grating-over-grating targets. In this paper, a number of
different imaging and scatterometry architectures are presented and compared in terms of random errors, systematic
errors and scribespace requirements. It is asserted that an optimal solution must combine the TMU peak performance
capabilities of scatterometry with the cost of ownership advantages of target size and multi-layer capabilities of imaging.
The overlay control budget for the 32nm technology node will be 5.7nm according to the ITRS. The overlay metrology
budget is typically 1/10 of the overlay control budget resulting in overlay metrology total measurement uncertainty
(TMU) requirements of 0.57nm for the most challenging use cases of the 32nm node. The current state of the art
imaging overlay metrology technology does not meet this strict requirement, and further technology development is
required to bring it to this level. In this work we present results of a study of an alternative technology for overlay
metrology - Differential signal scatterometry overlay (SCOL). Theoretical considerations show that overlay technology
based on differential signal scatterometry has inherent advantages, which will allow it to achieve the 32nm technology
node requirements and go beyond it. We present results of simulations of the expected accuracy associated with a
variety of scatterometry overlay target designs. We also present our first experimental results of scatterometry overlay
measurements, comparing this technology with the standard imaging overlay metrology technology. In particular, we
present performance results (precision and tool induced shift) and address the issue of accuracy of scatterometry
overlay. We show that with the appropriate target design and algorithms scatterometry overlay achieves the accuracy
required for future technology nodes.
Bright field imaging based metrology performance enhancement is essential in the quest to meet lithography process control requirements below 65 nm half pitch. Recent work has shown that, in parallel to the lithographic processes themselves, the metrology tools are able to continue to perform despite the fact that the size of the features under test are often below the classical Rayleigh resolution limit of the optical system. Full electromagnetic simulation is a mandatory tool in the investigation and optimization of advanced metrology tool and metrology target architectures. In this paper we report on imaging simulations of overlay marks. We benchmark different simulation platforms and methods, focusing in particular on the challenges associated with bright-field imaging overlay metrology of marks with feature sizes below the resolution limit. In particular, we study the dependence of overlay mark contrast and information content on overlay mark pitch and feature size.