As Extreme Ultraviolet Lithography (EUVL) enters the pre-production phase, the need to qualify the Electronic Design
Automation (EDA) infrastructure is pressing. In fact, it is clear that EUV will require optical proximity correction
(OPC), having its introduction shifted to more advanced technology nodes. The introduction of off-axis illumination will
enlarge the optical proximity effects, and EUV-specific effects such as flare and shadowing have to be fully integrated in
the correction flow and tested.
We have performed a model calibration exercise on the ASML NXE:3100 pre-production EUVL scanner using Brion's
Tachyon NXE EUV system. A model calibration mask has been designed, manufactured and characterized. The mask
has different flare levels, as well as model calibration structures through CDs and pitch. The flare modulation through
the mask is obtained by varying tiling densities. The generation of full-chip flare maps has been qualified against
experimental results. The model was set up and calibrated on an intermediate flare level, and validated in the full flare
Wafer data have been collected and were used as input for model calibration and validation. Two-dimensional structures
through CD and pitch were used for model calibration and verification. We discuss in detail the EUV model, and analyze
its various components, with particular emphasis to EUV-specific phenomena such as flare and shadowing.
EUVL requires the use of reflective optics including a reflective mask. The mask consists of an absorber layer pattern on
top of a reflecting multilayer, tuned for 13.53 nm. The EUVL mask is a complex optical element with many parameters
contributing the final wafer image quality. Specifically, the oblique incidence of light, in combination with the small
ratio of wavelength to mask topography, causes a number of effects which are unique to EUV, such as an HV CD offset.
These so-called shadowing effects can be corrected by means of OPC, but also need to be considered in the mask stack
In this paper we will present an overview of the mask contributors to imaging performance at the 27 nm node and below,
such as CD uniformity, multilayer and absorber stack composition, thickness and reflectivity. We will consider basic
OPC and resulting MEEF and contrast. These parameters will be reviewed in the context of real-life scanner parameters
both for the NXE:3100 and NXE:3300 system configurations.
The predictions will be compared to exposure results on NXE:3100 tools, with NA=0.25 for different masks. Using this
comparison we will extrapolate the predictions to NXE:3300, with NA=0.33.
Based on the lithographic investigation, expected requirements for EUV mask parameters will be proposed for 22 nm
node EUV lithography, to provide guidance for mask manufacturers to support the introduction of EUV High Volume
In this paper we will present ASML's holistic approach to lithography for EUV. This total approach combines the
various components needed to achieve the correct on-product demands of our customers in terms of patterning fidelity
across the entire image field and across the entire wafer.
We will start giving a general update on ASML's NXE scanner platform of which the 6<sup>th</sup> NXE:3100 systems is now
being shipped to a leading chipmaker. The emphasis will be on wafer imaging results for various applications such as
flash memory and logic's SRAM. Then we will describe the second holistic component, NXE-computational
lithography, which was developed to speed-up early learning on EUV and to achieve high accuracy on the wafers.
Thirdly, the YieldStar angular-resolved scatterometry tool that supports the scanner's stability was used to characterize
the system and calibrate the models.
The wafer-results reveal in detail predicted imaging effects of NXE lithography and allow a calibration of system
parameters and characterization of hardware components. We will demonstrate mask-induced imaging effects and
propose an improvement of the current EUV blank or mask-making processes.
EUVL requires the use of reflective optics including a reflective mask. The mask contains a reflecting
multilayer, tuned for 13.5 nm light, and an absorber which defines the dark areas. The EUV mask itself is a
complex optical element with many more parameters than just the mask CD uniformity of the patterned
features that impact the final wafer CDU. One of these parameters is absorber height. It has been shown
that the oblique incidence of light in combination with the small wavelength compared to the mask
topography causes a so-called shadowing effect manifesting itself particularly in an HV wafer CD offset. It
was also shown that this effect can be essentially decreased by reducing absorber height and, in addition, it
can be corrected by means of OPC.
However, reduction of absorber height has a side effect that is an increased reflectivity of a mask black
border resulting in field-to-field stray light due to parasitic reflections. One of the solutions to this problem
is optical process correction (OPC) at field edges. In this paper we will show experimental data obtained on
ASML EUV Alpha tool illustrating the black border effect and will demonstrate that this effect can be
accurately predicted by Brion Tachyon EUV model allowing for a significant cross field CD uniformity
improvement with mask layout correction technique.
Also we show by means of rigorous 3D simulations that it is possible to improve the imaging performance
significantly by performing global optimization of mask absorber height and mask bias in order to increase
exposure latitude, decrease CD sensitivity to mask making variations such as CD mask error and absorber
stack height variations. By sacrificing some exposure latitude throughput of exposure tool can be increased
essentially and HV mask biasing can be reduced. For four masks with different absorber thicknesses from
44 nm to 87 nm it is proven experimentally by means of the EUV Alpha tool exposures of 27 nm L/S that
the absorber thickness can be tuned to maximize exposure latitude. It was also proven that dose to size
grows with absorber height and optimal feature bias depends on mask absorber height.
The switch from 193i to EUV Photolithography will bring some fundamental changes in exposure. The flare levels of an
EUV machine are significantly higher compared with standard 193i machines. Moreover shadow effects on the reticle
are not equivalent to 193i. It is inevitable that these fundamentals require modifications in the Optical Proximity
Correction (OPC) flow.
In this paper in collaboration with ASML BRION the critical enabling steps of the Mask Data Preparation (MDP) for
EUVL, Flare, Shadowing and Optical and Process Corrections (OPC), are investigated.
We measured the needs of the EUV MDP flow against the capabilities of a state-of-the art OPC flow built for 193i.
Adaptations are being made to implement features which are currently not available in a 193i based flow.
We present a feasibility study of the Model Based approach to the EUV OPC on a wide selection of features. Also we
demonstrate simulations and verification of the EUV modeling capabilities of the TachyonTM with various levels and
ranges of flare and prove the applicability of the reviewed approach to the process development for the 27nm EUV
We also evaluated the accuracy of the EUV OPC modeling and expected OPC corrections on the reviewed selection of
clips as a substantial part of the overall CDU budget.
Finally an overall EUV OPC flow as a manufacturable solution based on the Tachyon's predictions and ASML's
knowledge of Photolithography was discussed.
Accurate modeling of EUV Lithography is a mandatory step in driving the technology towards its foreseen insertion
point for 22-16nm node patterning. The models are needed to correct EUV designs for imaging effects, and to
understand and improve the CD fingerprint of the exposure tools. With a full-field EUV ADT from ASML now
available in the IMEC cleanroom, wafer data can be collected to calibrate accurate models and check if the existing
modeling infrastructure can be extended to EUV lithography. As a first topic, we have measured the CD on wafer of a
typical OPC dataset at different flare levels and modeled the evolution of wafer CD through flare, reticle CD, and pitch
using Brion's Tachyon OPC engine. The modeling first requires the generation of a flare map using long-range kernels
to model the EUV specific long-range flare. The accuracy of the flare map can be established independently from the CD
measurements, by using the traditional disappearing pad test for flare determination (Kirk test). The flare map is then
used as background intensity in the calibration of the traditional optical models with short-range kernels. For a structure
set of 600 features and over a flare range of 4-6%, an rms fit value of 0.9nm was obtained.
As a second aspect of the modeling, we have calibrated a full resist model to process window data. The full resist model
is then used in a combination with experimental measurements of reticle CD, slit intensity uniformity, focal plane
behavior, and EUV thick mask effects to model the evolution of wafer CD across the exposure field. The modeled
evolution of CD across the exposure field was found to be a good match to the experimentally seen evolution of CD
across the field, and confirms that the 4 factors mentioned above are main contributions to the CD uniformity across the
field. As such the modeling work enables a better understanding of the errors contributing to CD variation across the
field for EUV technology.
The challenge for the upcoming full-chip CD uniformity (CDU) control at 32nm and 22nm nodes is unprecedented with
expected specifications never before attempted in semiconductor manufacturing. To achieve these requirements, OPC
models not only must be accurate for full-chip process window characterization for fine-tuning and matching of the
existing processes and exposure tools, but also be trust-worthy and predictive to enable processes to be developed in
advance of next-generation photomasks, exposure tools, and resists. This new OPC requirement extends beyond the
intended application scope for behavior-lumped models. Instead, separable OPC models are better suited, such that each
model stage represents the physics and chemistry more completely in order to maintain reliable prediction accuracy. The
resist, imaging tool, and mask models must each stand independently, allowing existing resist and mask models to be
combined with new optics models based on exposure settings other than the one calibrated previously.
In this paper, we assess multiple sets of experimental data that demonstrate the ability of the Tachyon<sup>TM</sup> FEM (focus and
exposure modeling) to separate the modeling of mask, optics, and resists. We examine the predictability improvements
of using 3D mask models to replace thin mask model and the use of measured illumination source versus top-hat types.
Our experimental wafer printing results show that OPC models calibrated in FEM to one optical setting can be
extrapolated to different optical settings, with prediction accuracy commensurate with the calibration accuracy. We see
up to 45% improvement with the measured illumination source, and up to 30% improvement with 3D mask.
Additionally, we observe evidence of thin mask resist models that are compensating for 3D mask effect in our wafer data
by as much as 60%.
A new framework has been developed to model 3D thick mask effects for full-chip OPC and verifications. In addition to
electromagnetic (EM) scattering effects, the new model also takes into account the non-Hopkins oblique incidence
effects commonly found in real lithography systems but missing in prior arts. Evaluations against rigorous simulations
and experimental data showed the new model provides improved accuracy, compared to both the thin-mask model and
the thick-mask model based on Hopkins treatment of oblique incidence.