Next-generation metrology solutions in various technology areas require to image sample areas at the nanoscale. Coherent diffractive imaging based on ptychography is the route towards EUV imaging of nanostructures without lenses. A key component in a table-top EUV beamline is a high-brightness high-harmonic generation (HHG) source. Since our research is mainly directed towards wafer metrology for lithography in the semiconductor industry, we adhere to a reflection setup: the EUV light is scattered by the nanostructures at the surface of the sample, and is reflected towards a CCD camera, where a far-field diffraction pattern is recorded. A data-set comprising a multitude of these diffraction patterns is generated for partially overlapping positions of the focused probe on the sample. This provides the necessary redundancy for phase retrieval of the complex-valued field of the sample. Recent advancements in both hardware and software for computation enable the development of advanced algorithms. In particular, the benefits of automatic differentiation are exploited in order to cope with a drastic growth in model complexity. Our computational imaging algorithms realize wavelengthmultiplexed reconstruction and a modal approach for the spatial coherence of the source.
Ptychography as a means of lensless imaging is used in wafer metrology applications using Extreme Ultraviolet (EUV) light, where use of high quality optics is out-of-scope. To obtain sufficient diffraction intensity, reflection geometries with shallow (ca. 20 degrees) grazing incidence angles are used, which require re-sampling the diffraction data in a process called tilted plane correction (TPC). The tilt angle used for TPC is conventionally obtained through either experimentally tricky calibration, manual estimation based on diffraction pattern symmetry, although computational approaches are emerging. In this work we offer an improved numerical optimization approach as an alternative to TPC, where we use the flexibility offered by our Automatic Differentiation (AD)-based ptychography approach to include the data resampling into the forward model to learn the tilt angle. We demonstrate convergence of the approach across a range of incidence angles on simulated and experimental data obtained on an EUV beamline with either a high-harmonic generation (HHG)-based or a visible light source.
The demand for accurate and sub-nanometer precise overlay measurements in semiconductor industry increases with the shrinking feature sizes in integrated circuits. Overlay, the lateral displacement between two layers allows monitoring the chip fabrication process and is part of an import feedback step. In our approach, a digital holographic microscope measures the complex field of the overlay target using simple optics and an additional reference beam. The measured complex valued field allows us to apply computational algorithms to correct for field-position dependent lens aberrations in a computational efficient manner. We present experimental results that show the capability of our computational aberration correction in the visible and near infrared wavelength regimes.
High harmonic generation at high repetition rate is realized with a high average power 100W, 600kHz fiber laser system. Optimization is done for two different operation regimes. At 69-75eV the source delivers a world-record photon flux of >10^11photons/s/harmonic when using argon gas jets. The use of neon gas allows for operation at significantly shorter wavelength. The important 93eV harmonic can be generated at 5·10^9 photons/s/(1% bandwidth), while even higher values of >10^10 photons/s/(1% bandwidth) are achieved between 115-140eV. The HHG source provides excellent long-term power stability of ~1% RMS for each of the operation regimes.
The imaging and inspection of extreme ultraviolet (EUV) masks is an important aspect of EUV lithography. The availability of actinic mask inspection tools able to generate highly resolved defect maps of defective EUV layouts is needed to ensure defect–free wafer prints. The technological interest towards phase–shift absorber materials for the next generation of EUV masks, and the associated need for phase metrology at the absorber level, makes phase retrieval methods a particularly interesting option for actinic inspection. In this work we use ptychography as an inspection tool for EUV masks. We show how variational and statistical methods can be employed to include a–priori information in the ptychographic inverse problem and how to cluster different update rules – stemming from the minimization of appropriate cost functionals – to optimally include prior information in ptychography under Poisson noise.
EUV lithography is the main candidate for patterning of future technology nodes. Its successful implementation depends on many aspects, among which the availability of actinic mask metrology tools able to inspect the patterned absorber in order to control and monitor the lithographic process. In this work, we perform a simulation study to assess the performance of coherent diffractive imaging (CDI) and related phase retrieval methods for the reconstruction of non-trivially shaped and a–periodic nanostructures from far field intensity data.
We have designed a ptychographical algorithm (HIO-PIE) in which HIO (Hybrid Input-Output) iterations are applied for each probe position sequentially. Simulations indicated that HIO-PIE tends to outperform PIE (Ptychographical Iterative Engine) even in the presence of shot noise, especially when the number of probe positions is small. Thus, this adaptation may reduce the number of scan positions required for a successful image reconstruction, thereby relaxing the constraints on an experimental setup. In this article, experimental results obtained using an SLM (spatial light modulator) and visible laser light (633 nm) show that when few probe positions are used, HIO-PIE converges significantly faster than PIE
The imaging quality of the projection optics of an extreme ultraviolet lithography scanner degrades under the influence of thermally induced deformations of its mirrors. Wavefronts of different reticle points encounter different parts of the deformed mirrors, resulting in a field dependent wavefront error. This paper presents how ideas from multi-conjugate adaptive optics can be used to reduce these thermally induced aberrations. To this end a generic deformable mirror model is implemented. Linear actuator sensitivities are derived directly, based on nominal ray locations and directions, enabling fast prototyping. An integrated opto-thermo-mechanical mirror heating model is used to determine the evolution of thermally induced abberations over time. This transient simulation is used to analyze four different adaptive optics configurations and two different control algorithms. It is shown that by employing the multi-objective goal-attainment method, it is possible to improve the optical performance significantly when compared to minimizing the ℓ2-norm of the total residual wavefront error vector.
Incoherent Optical Scatterometry (IOS) is a well-established metrology technique in the semiconductor industry to
retrieve periodic grating structures with high accuracy from the signature of the diffracted optical far field. With
shrinking dimensions in the lithography industry, finding possible improvements in wafer metrology is highly desirable.
The grating is defined in terms of a finite number of geometrical shape parameters (height, side-wall angles, midCD
etc.). In our method the illumination is a scanning focused spot from a spatially coherent source (laser) within a single
period of the grating. We present a framework to study the increment in sensitivity of Coherent Fourier Scatterometry
(CFS) with respect to the IOS. Under suitable conditions, there is a more than fourfold enhancement in sensitivity for
grating shape parameters using CFS. The dependence of scanning positions on the sensitivity analysis is also highlighted.
We further report the experimental implementation of a Coherent Fourier Scatterometer. The simulated and
experimental far fields are compared and analyzed for the real noise in the experimental configuration.
A brand new CD metrology technique that can address the need for accuracy, precision and speed in near future
lithography is probably one of the most challenging items. CDSEMs have served this need for a long time,
however, a change of or an addition to this traditional approach is inevitable as the increase in the need for better
precision (tight CDU budget) and speed (driven by the demand for increase in sampling) continues to drive the
need for advanced nodes.
The success of CD measurement with scatterometry remains in the capability to model the resist grating, such as,
CD and shape (side wall angle), as well as the under-lying layers (thickness and material property). Things are
relatively easier for the cases with isotropic under-lying layers (that consists of single refractive or absorption
indices). However, a real challenge to such a technique becomes evident when one or more of the under-lying
layers are anisotropic.
In this technical presentation the authors would like to evaluate such CD reconstruction technology, a new
scatterometry based platform under development at ASML, which can handle bi-refringent non-patterned layers
with uniaxial anisotropy in the underlying stack. In the RCWA code for the bi-refringent case, the elegant
formalism of the enhanced transmittance matrix can still be used. In this paper, measurement methods and data
will be discussed from several complex production stacks (layers). With inclusion of the bi-refringent modeling,
the in-plane and perpendicular n and k values can be treated as floating parameters for the bi-refringent layer, so
that very robust CD-reconstruction is achieved with low reconstruction residuals. As a function of position over
the wafer, significant variations of the perpendicular n and k values are observed, with a typical radial fingerprint
on the wafer, whereas the variations in the in-plane n and k values are seen to be considerably lower.
For the optical recording channel, a new runlength-limited (RLL) code is proposed, called the 1102PC code, which is a d=1 RLL code with a compact 2-to-3 PCWA-mapping for DC-control that offers a significant performance benefit over traditional soft-decodable d=1, k=7 codes thanks to the reduced frequency of occurrence of the shortest 2T-runs as realized via an r=2 RMTR-constraint; in addition, it still offers a low k-constraint (k=10) and a SISO-complexity much lower than that of 17PP. The 1102PC code has also been evaluated experimentally for near-field optical recording.
KEYWORDS: Sensors, Receivers, Optical storage, Signal processing, Compact discs, Electronic filtering, Distortion, Signal detection, Near field optics, Digital video discs
This paper discusses several issues related to adaptation and timing recovery for 2D Optical Storage. PRML detection in the form of a stripe-wise Viterbi detector is used. A 2D equalizer is applied to transform the channel response to a target response with limited span.
KEYWORDS: Stochastic processes, Computer programming, Digital video recorders, Information operations, Digital video discs, Lithium, Optical recording, Optical discs, Video, Modulation
EFMCC is a runlength-limited (RLL) channel code with EFM- like RLL constraints, d equals 2 and k equals 10, which is constructed by combining two codes, a main code and a substitution code. Both codes operate on a byte-by-byte basis. The substitution code has a special structure, i.e., for each byte, there are two possible channel words, which have opposite parity and the same next-state in the finite- state machine of the EFMCC code. The benefits are: guaranteed DC-control, 4% higher efficiency than EFMPlus, and simple byte-oriented look-ahead DC-control encoding.
For the DVR system with the use of a blue laser diode (wavelength 405 nm) we developed (12 cm) discs with a total capacity of 22.4 GB. The land/groove track pitch is 0.30 micrometers and the channel bit length is 87 nm. The DVR system uses a d equals 1 code. These phase change discs can be recorded at continuous angular velocity at a maximum of 50 Mbps user data rate (including all format and ECC overhead) and meet the system specifications. Fast growth determined phase change materials (FGM) are used for the active layer. In order to apply these FGM discs at small track pitch special attention has been paid to the issue of thermal cross-write. Finally routes towards higher capacities such as advanced bit detection schemes and the use of a smaller track pitch are considered. These show the feasibility in the near future of at least 26.0 GB on a disc for the DVR system with a blue laser diode.
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