Extreme ultraviolet lithography (EUVL) systems struggle from both low source brightness and low source throughput through the tool. For these reasons, photon shot noise will play a much larger role in image process development for EUVL than in DUV processes. Furthermore, the lower photon count increases the stochastic variation of all the processes which occur after photon absorption. This causes the printed edge to move away from the mean edge with some probability. This paper will present a model form and calibration flow for including stochastic probability bands in compact models suitable for full chip simulation. This model form relies on calibrating to statistical data from a rigorous EUV stochastic lithography model calibrated to wafer experimental data. The data generation, data preparation, and model calibration flows for the compact stochastic probability bands will be presented. We will show that this model form can predict patterns which are prone to stochastic pattern failure in realistic mask designs, as well as how this model form can be used downstream for full chip correction (e.g., SMO, OPC and/or ILT).
In EUV lithography, the short wavelength and residual mirror surface roughness increase the flare levels across the slit. As a key research point, the flares of different exposure fields are carefully discussed by numerical simulation. To ensure the effectiveness and practicability of our simulations, the test patterns are generated according to the general design rules for 7nm technology node. The NILS, process variation band (PVB) and MEEFs from mask optimizations and source mask optimizations (SMO) results are compared. From the comparisons, the constant flare has a greater influence on NILS and PVB than that on MEEF. In contrast, the flare map caused more reduction on the MEEF values.
Aberrations must be sufficiently controlled to make moving to a higher numerical aperture worthwhile. Traditional isomorphic imaging systems form the same image regardless of their rotation. Likewise, the aberration basis chosen for isomorphic optics is invariant under rotation. Anamorphic optics are not rotationally invariant though—they are only reflection invariant. We have shown in previous reports that a basis composed from a product of Legendre polynomials represents the balanced aberrations of anamorphic optics. Solutions have been presented under the presence of a circular central obscuration. This paper will examine the properties of these aberrations and their effects on image formation through analogies to the well-known Zernike aberrations. It will be shown that ray tracing simulations of the point spread function of an anamorphic optic in Code V matches predictions made by the proposed basis. A system will be described for computing an anamorphic aberration basis in the presence of an arbitrary obscuration. Based on this system we will analyze the effects of using the basis for the wrong type of obscuration.
As the power of laser produced plasma sources has increased, EUV lens heating has become a major component of process variation. Differential lens heating can cause thermal aberrations which affect system drift during operation, therefore pupil plane characterization will play a critical role in process optimization for EUV lithography (EUVL). In-situ full pupil characterization, which depicts the pupil in its in-use state, is essential for these tools. To this end we have developed Quick Inverse Pupil (QUIP)—a software suite developed for rapid characterization of pupil plane behavior based on images formed by that system. This algorithm is based on statistical modeling, which correlates image-space variation with known aberrations. Previously we have presented variations on this algorithm which can only measure third-order aberrations and requires aerial image data. In this paper, we will present an approach to measure high order aberrations from images formed in resist. An inverse pupil solution will be obtained from CD-SEM image analysis. We will show that the additional degrees of freedom required to measure high-order aberrations can be achieved through using different pitches of the metrology targets. We will demonstrate that this technique can accurately determine third- and fifth-order aberrations with a retrieval error below 0.5 miliwaves in under one second. A combination of synthetic and experimental data will be presented.
Aberration characterization plays a critical role in the development of any optical system. State-of-the-art lithography systems have the tightest aberration tolerances. We present an approach to image-based pupil plane amplitude and phase characterization using models built with a space-domain basis, in which aberration effects are separable. A polynomial model is constructed between the projections of the image intensity for chosen binary mask targets onto this basis and pupil amplitude or phase variation. This method separates model building and pupil characterization into two distinct steps, thus enabling rapid pupil characterization following data collection. The basis is related to both the transmission cross-coefficient function and the principal components of the image intensity. The pupil plane variation of a zone-plate lens from the Semiconductor High-NA Actinic Reticle Review Project (SHARP) at Lawrence Berkeley National Laboratory is examined using this method. Results are compared to pupil plane characterization using a previously proposed methodology where inverse solutions are obtained through an iterative process involving least-squares regression.
Next-generation EUV lithography systems will use anamorphic optics to achieve high-NA. The well-known Zernike circle polynomials do not describe the sixteen primary aberrations of these anamorphic optical systems though. We propose to use a basis which does describe the primary aberrations. We examine the properties of this new basis and how they impact lithographic processes through analogies to isomorphic aberrations. We have developed an application to use the proposed basis in existing lithography simulators. There is an additional importance in EUVL placed on understanding how pupil variation evolves during system operation. Interferometric methods are the de facto standard of pupil phase metrology but are challenging to implement during tool use. We have previously presented an approach to measure both the pupil amplitude and phase variation of isomorphic EUVL systems from images formed by that system. We show how this methodology can be adapted to anamorphic optical systems. More specifically, we will present a set of binary metrology targets sensitive to the anamorphic primary aberrations.
An approach to image-based EUV aberration metrology using binary mask targets and iterative model-based solutions to extract both the amplitude and phase components of the aberrated pupil function is presented. The approach is enabled through previously developed modeling, fitting, and extraction algorithms. We seek to examine the behavior of pupil amplitude variation in real-optical systems. Optimized target images were captured under several conditions to fit the resulting pupil responses. Both the amplitude and phase components of the pupil function were extracted from a zone-plate-based EUV mask microscope. The pupil amplitude variation was expanded in three different bases: Zernike polynomials, Legendre polynomials, and Hermite polynomials. It was found that the Zernike polynomials describe pupil amplitude variation most effectively of the three.
Pupil plane characterization will play a critical role in image process optimization for EUV lithography (EUVL), as it has for several lithography generations. In EUVL systems there is additional importance placed on understanding the ways that thermally-induced system drift affect pupil variation during operation. In-situ full pupil characterization is therefore essential for these tools. To this end we have developed Quick Inverse Pupil (QUIP)—a software suite developed for rapid characterization of pupil plane behavior based on images formed by that system. The software consists of three main components: 1) an image viewer, 2) the model builder, and 3) the wavefront analyzer. The image viewer analyzes CDSEM micrographs or actinic mask micrographs to measure either CDs or through-focus intensity volumes. The software is capable of rotation correction and image registration with subpixel accuracy. The second component pre-builds a model for a particular imaging system to enable rapid pupil characterization. Finally, the third component analyzes the results from the image viewer and uses the optional pre-built model for inverse solutions of pupil plane behavior. Both pupil amplitude and phase variation can be extracted using this software. Inverse solutions are obtained through a model based algorithm which is built on top of commercial rigorous full-vector simulation software.
We present an approach to image-based pupil plane amplitude and phase characterization using models built with principal component analysis (PCA). PCA is a statistical technique to identify the directions of highest variation (principal components) in a high-dimensional dataset. A polynomial model is constructed between the principal components of through-focus intensity for the chosen binary mask targets and pupil amplitude or phase variation. This method separates model building and pupil characterization into two distinct steps, thus enabling rapid pupil characterization following data collection. The pupil plane variation of a zone-plate lens from the Semiconductor High-NA Actinic Reticle Review Project (SHARP) at Lawrence Berkeley National Laboratory will be examined using this method. Results will be compared to pupil plane characterization using a previously proposed methodology where inverse solutions are obtained through an iterative process involving least-squares regression.
Proc. SPIE. 9776, Extreme Ultraviolet (EUV) Lithography VII
KEYWORDS: Lithography, Diffraction, Monochromatic aberrations, Point spread functions, Deep ultraviolet, 3D modeling, Photomasks, Extreme ultraviolet, Extreme ultraviolet lithography, Systems modeling, 3D image processing, Phase shifts
The non-zero chief ray angle at the object (CRAO) in EUVL systems introduces azimuthally asymmetric phase shifts. Understanding and characterizing these effects is critical to EUVL system and mask design. The effects of 3D mask absorber geometry on diffraction phase were examined through rigorous simulation. The diffraction phase distribution was split into even and odd components to enable analogies between the well-known effects of lens aberrations and EUV 3D mask effects. Specifically, this analysis reveals that the odd component of the phase distribution is non-zero in off-axis optical systems. We have found that 3D mask effects in EUVL systems can be partially compensated in the pupil plane to minimize aerial image effects, such as best focus shifts, horizontal-vertical CD bias, and image placement error.
We present an approach to image-based EUV aberration metrology using binary mask targets and iterative model-based solutions to extract both the amplitude and phase components of the aberrated pupil function. The approach is enabled through previously developed modeling, fitting, and extraction algorithms. We examine the flexibility and criticality of the method using two experimental case studies. The first extracts the pupil phase behavior from an ASML NXE:3100 exposure system and shows primary aberration sensitivity below 0.2 mλ. The second experiment extracts both components of the pupil function from the SHARP EUV microscope.
As EUV lithography attempts to outperform other lithographical methods to the sub-14 nm node, the demand for a larger NA traditionally dominates the drive for scaling. There are, however, many challenges to overcome in order to accomplish this . Due to the reflective optics in EUV systems, angular effects of oblique illumination, and non-zero chief ray angle at the objective (CRAO), must be carefully considered and will need to be well understood if high-NA EUV is to be successful. This study investigates impact on of the bias between horizontal and vertical feature CD, image placement error and NILS. Effects of sidewall absorber angle, absorption coefficient (k) and absorber thickness are observed through pitch with various source shapes in an EUV lithography system.
EUV lithography is likely more sensitive to drift from thermal and degradation effects than optical counterparts. We have developed an automated approach to photoresist image-based aberration metrology. The approach uses binary or phase mask targets and iterative simulation based solutions to retrieve an aberrated pupil function. It is well known that a partially coherent source both allows for the diffraction information of smaller features to be collected by the condenser system, and introduces pupil averaging. In general, smaller features are more sensitive to aberrations than larger features, so there is a trade-off between target sensitivity and printability. Therefore, metrology targets using this technique must be optimized for maximum sensitivity with each illumination system. This study examines aberration metrology target optimization and suggests an optimization scheme for use with any source. Interrogation of both low and high order aberrations is considered. High order aberration terms are interrogated using two separate fitting algorithms. While the optimized targets do show the lowest RMS error under the test conditions, a desirable RMS error is not achieved by either high order interrogation scheme. The implementation of a previously developed algorithm for image-based aberration metrology is used to support this work.
The roughness present on the sidewalls of lithographically defined patterns imposes a very important challenge for advanced technology nodes. It can originate from the aerial image or the photoresist chemistry/processing . The latter remains to be the dominant group in ArF and KrF lithography; however, the roughness originating from the mask transferred to the aerial image is gaining more attention [2-9], especially for the imaging conditions with large mask error enhancement factor (MEEF) values. The mask roughness contribution is usually in the low frequency range, which is particularly detrimental to the device performance by causing variations in electrical device parameters on the same chip [10-12]. This paper explains characteristic differences between pupil plane filtering in amplitude and in phase for the purpose of mitigating mask roughness transfer under interference-like lithography imaging conditions, where onedirectional periodic features are to be printed by partially coherent sources. A white noise edge roughness was used to perturbate the mask features for validating the mitigation.
Historically IC (integrated circuit) device scaling has bridged the gap between technology nodes. Device size reduction
is enabled by increased pattern density, enhancing functionality and effectively reducing cost per chip. Exemplifying
this trend are aggressive reductions in memory cell sizes that have resulted in systems with diminishing area between
bit/word lines. This affords an even greater challenge in the patterning of contact level features that are inherently
difficult to resolve because of their relatively small area and complex aerial image. To accommodate these trends,
semiconductor device design has shifted toward the implementation of elliptical contact features. This empowers
designers to maximize the use of free device space, preserving contact area and effectively reducing the via dimension
just along a single axis. It is therefore critical to provide methods that enhance the resolving capacity of varying aspect
ratio vias for implementation in electronic design systems. Vortex masks, characterized by their helically induced
propagation of light and consequent dark core, afford great potential for the patterning of such features when coupled
with a high resolution negative tone resist system. This study investigates the integration of a vortex mask in a 193nm
immersion (193i) lithography system and qualifies its ability to augment aspect ratio through feature density using aerial
image vector simulation. It was found that vortex fabricated vias provide a distinct resolution advantage over
traditionally patterned contact features employing a 6% attenuated phase shift mask (APM). 1:1 features were
resolvable at 110nm pitch with a 38nm critical dimension (CD) and 110nm depth of focus (DOF) at 10% exposure
latitude (EL). Furthermore, iterative source-mask optimization was executed as means to augment aspect ratio. By
employing mask asymmetries and directionally biased sources aspect ratios ranging between 1:1 and 2:1 were
achievable, however, this range is ultimately dictated by pitch employed.