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Alternatives to Ta-based absorbers are being considered for next generation lithography nodes to reduce 3D mask effects and to improve image modulation through phase interference. Low complex refractive index (n – ik) materials can provide phase shifting behavior at thicknesses less than those needed for conventional absorbers, essentially acting as attenuated phase shift mask (attPSM) films. Identifying attPSM absorber thickness and consequent phase requires determining optimum phase shift mask reflectance. Imaging with absorbers at high reflectance show better imaging performance. The absorber thickness is determined where the interference effects lead to high absorber reflectivity. Low refractive index (n) materials are therefore desired as candidate attPSM absorbers. Low – n material combinations identified using Wiener bounds and Effective media approximation (EMA) modelling are optimized for NILS and MEEF using absorber reflectivity on line-space and contact-holes patterns. Absorber candidates at optimum thickness for contact holes are compared with conventional Ta-based absorber using reflected nearfield intensity imaging.

EXPOSURE SYSTEM System Overview of Optical Steppers and Scanners Michael S. Hibbs Optical Lithography Modeling Chris A. Mack Optics for Photolithography Bruce W. Smith Excimer Laser for Advanced Microlithography Palash Das Alignment and Overlay Gregg M. Gallatin Electron Beam Lithography System Kazuaki Suzuki X-Ray Lithography Takumi Ueno EUV Lithography Stefan Wurm and Charles Gwyn Imprint Lithography Douglas J. Resnick RESISTS AND PROCESSING Chemistry of Photoresist Materials Takumi Ueno and Robert D. Allen Resist Processing Bruce W. Smith Multilayer Resist Technology Bruce W. Smith and Maureen Hanratty Dry Etching of Photoresists Roderick R. Kunz METROLOGY AND NANOLITHOGRAPHY Critical-Dimensional Metrology for Integrated-Circuit Technology Herschel M. Marchman, Gian Lorusso, Mike Adel, and Sanjay Yedur Electron Beam Nanolithography Elizabeth A. Dobisz, Zvonimir Z. Bandic, and Martin C. Peckerar Index

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 extreme ultraviolet lithography (EUVL) technology progresses towards and below sub-7nm generations, polarization effects will begin to have an impact. Moving to higher NA will introduce polarization effects at all locations: the mask plane, the optics, and the image plane. Contrast losses at a larger angle can occur from the interference at the image plane that will become significant for smaller geometries approaching the EUVL wavelength. Some of these contrast losses can occur at the mask where the polarized reflectance from an EUV mask is affected by the multilayer reflective stack and patterned features. This paper explores the polarization effects that are induced by EUVL masks for sub-7nm lithography. The results show a polarization-dependent induction and attenuation of current in EUVL mask structures, especially as mask pitch decreases below 6λ.

Pattern transfer in an extreme ultraviolet lithography (EUVL) system requires reflective optical elements illuminated at oblique illumination angles. This, in combination with the three-dimensional effects at the mask, is the source of the so-called mask 3D (M3D) effects that include shadowing, best focus shifts, and contrast fading. Alternative mask absorbers at lower thickness possess the ability to extenuate the M3D effects and improve overall imaging performance. An approach to recognize candidate material combinations as alternative EUVL mask absorbers through dielectric constant modeling by using methods involving Wiener bounds and effective media approximation (EMA) is presented. Using EMA, several stoichiometrically stable binary alloys of refractory metals that may serve as attenuated phase shifting mask absorbers have been studied. The optical properties and thickness of each absorber candidate alloy in a standard EUV mask stack are optimized for imaging performance. The best performing alloys are characterized through rigorous 3D image modeling of near-field intensity and phase at varied illumination angles.

As the extreme ultraviolet (EUV) lithography technology progresses towards and below sub-7nm generations, polarization effects will begin to have an impact. As numerical apertures increase, the consequences at both the mask and the wafer plane need to be understood. Contrast losses at large angles can occur from non-ideal interference at the wafer plane (i.e. TM vs. TE polarization). While such loss of infidelity can be low for near wavelength half-pitch generations, additional image degradation can be attributed to polarization effects for higher resolution generations. Some of this arises from the mask. The polarized reflectance from a EUV photomask is influenced by the multilayer reflective stack as well as the polarizing effect of the patterned features. This paper explores the polarization effects that are induced by EUV masks for sub-7nm lithography generations. From the results, it was found that there is polarization-dependent induction and attenuation of current in EUV mask structures as mask pitch decreases.

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.

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.

© 2016 Society of Photo-Optical Instrumentation Engineers (SPIE). 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.

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.

The degradation of projected images using TM polarization is not intrinsic because losses in image contrast can be recoverable. By controlling the photoresist/substrate interface reflectivity, high modulation for TM polarization can be maintained for angles approaching 90° in a photoresist. Although there is calculated loss of image contrast with increasing polarization angle using suppressed reflection (i.e., with an antireflection coating), the loss is not nearly as large when imaging over a reflective substrate. These results can potentially impact the design of illumination, possibly away from most recent TE-only schemes for oblique imaging angles and high numerical apertures (NA). Several cases of TM illumination are presented combined with tuned substrate reflectivity for 0.93NA, 1.20NA, and 1.35NA and compared to results using TE illumination. Additionally, a scheme for frequency doubling with a single TM polarized exposure is presented. Using a single exposure and selective TM polarization, the re...

In recent years, the anomalous transmission of subwavelength apertures has become an emergent subject within the physical sciences. While the gain mechanism of these structures is still uncertain, the effect has been observed in several studies. Similar transmission enhancements may be realized for near-wavelength sized photomask structures by including buried grooves in a dual write mask design. Several configurations of one-dimensional transmitting apertures and buried grooves have been investigated under TM illumination using the finite element method. Periodic subwavelength apertures with identical sized nontransmitting assist grooves were used to validate the model against reference data in the near-IR. The method was also used to investigate similar structures featuring a larger than wavelength transmitting aperture. Although transmission through the slot was increased by 1.67×, the primary lithographic benefit was an increase in the magnitude of the primary imaging orders relative to the zeroth dif...

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.

© 2017 Society of Photo-Optical Instrumentation Engineers (SPIE). 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.

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.

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 [1]. 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.

As EUV Lithography (EUVL) continues to evolve, it offers a possible solution to the problems of additional masks and lithography steps that drive up the cost and complexity of 193i multiple patterning. EUVL requires a non-telecentric reflective optical system for operation. This requirement causes EUV specific effects such as shadowing. The absorber physically shadows the reflective multilayer (ML) on an EUV reticle resulting in pattern fidelity degradation. To reduce this degradation, a thinner absorber may help. Yet, as the absorber thickness decreases, reflectivity increases in the ‘dark’ region around the image field, resulting in a loss of contrast. The region around the edge of the die on the mask of unpatterned absorber material deposited on top of ML, known as the image border, is also susceptible to undesirable reflections in an ideally dark region. For EUVL to be enabled for high-volume manufacturing (HVM), reticle masking (REMA) blades are used to shield light from the image border to allow for the printing of densely spaced die. When die are printed densely, the image border of each neighboring die will overlap with the edge of a given die resulting in an increase of dose that overexposes features at the edge of the field. This effect is convolved with a fingerprint from the edge of the REMA blades. This phenomenon will be referred to as a field edge effect. One such mitigation strategy that has been investigated to reduce the field edge effect is to fully remove the ML along the image border to ensure that no actinic-EUV radiation can be reflected onto neighboring die. This has proven to suppress the effect, but residual out-of-band radiation still provides additional dose to features near the image border, especially in the corners where three neighboring fields overlap. Measurements of dense contact holes (CHs) have been made along the image border with and without a ML-etched border at IMEC in collaboration with Micron using the ASML NXE:3100. The implementation of these measurements allow for further mitigation, i.e., compensation by OPC. Mentor Graphics’ Calibre software uses the scanner’s point spread function and convolves it with the mask layout to generate a flare map. It also has the capability to add additional dose to the image border which can be optimized to fit the experimental data. This includes the transition region between the image field and border that results in a linear rolloff of dose due to partial shadowing of the REMA blades. By applying this flaremap that accounts for neighboring die to the already calibrated optical and resist models, OPC can now be enabled to compensate for field edge effects. This study has two goals. First, we will show that OPC can be used to compensate both for field edge effects with and without a etched ML border. The second is to investigate the limitations that exist for OPC in the areas altered by neighboring die. This will predict when a process to mitigate the field edge effect is needed to enable EUV HVM.

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 [1]. 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.

The optimization of illumination in optical lithography has been central to the progress toward λ/5 resolution now commonplace. The tailoring of source shapes to meet increasing demands of imaging has been commercially practiced for over twenty years. The influence of illumination in microscopy and micrography had been explored for centuries before an adequate description was developed in the late nineteenth century. By the mid-twentieth century, the mathematical foundation for the dependency of resolution on illumination was presented, leading to the parameterization of illumination through a partial coherence factor (σ). The role of partial coherence in microscopy, micrography, and microlithography has been critical to the extension of resolution in each of these fields and has allowed for the application of optical lithography toward sub-50nm resolution. Today, source customization, polarized illumination, and source/mask domain co-optimization have become critical tools used to achieve manufacturable lithography resolution at the edge of physical limits. This paper will provide a historical look into how illumination, partial coherence, and source shaping has influenced optical lithography. Aspects relative to future lithography generations are also explored, including illumination influences with 3D effects especially important for EUV wavelength applications.

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.

Copolymers of trimethysilylmethyl methacrylate and chloromethylstyrene [P(SI-CMS)] have been formulated as negative resists for exposure at 193 nm. To achieve maximum sensitivity, absorption has been targeted so that the amount of radiation exposing the base of a given film thickness is I 0 e −1 , leading to an optimum optical absorbance of log 10 e , or 0.434. Through control of the mole ratio of the monomers in the P(SI-CMS) copolymer, optimum response has been tailored for coating thicknesses from 2000 A to 5500 A. Optimal formulations yield working sensitivities of from 4 to 20 mJ/cm 2 for materials having a M w of 4 × 10 4 g/mole, with resolution demonstrated below 0.4 μm. Resists exhibit etching resistance in O 2 RIE and are suitable for application in both single layer and bi-layer processes.

As semiconductor nanolithography is pushed to smaller dimensions, process yields can suffer from three dimensional sub-wavelength imaging effects. Mask topography can influence the propagating diffracted field causing errors such as pitch dependent defocus and degraded useable depth of focus (UDOF). In this work, the compensation of diffracted phase error is realized through the manipulation of the wavefront in the projection lens pupil. Mask exposure data is presented showing how such manipulation can increase the UDOF for several mask structure types, leading to UDOF improvement between 18% and 83%. An analytical model is presented to understand trends seen in experimental data through pitch. Results also show that an asymmetric wavefront can be tuned to particular geometries, providing a UDOF improvement for line ends under restricted processing conditions.

Line edge roughness (LER) is a common problem to most lithography approaches and is seen as the main resolution limiter for advanced technology nodes1. There are several contributors to LER such as chemical/optical shot noise, random nature of acid diffusion, development process, and concentration of acid generator/base quencher. Since interference-like lithography (IL) is used to define one directional gridded patterns, some LER mitigation approaches specific to IL-like imaging can be explored. Two methods investigated in this work for this goal are (i) translational image averaging along the line direction and (ii) pupil plane filtering. Experiments regarding the former were performed on both interferometric and projection lithography systems. Projection lithography experiments showed a small amount of reduction in low/mid frequency LER value for image averaged cases at pitch of 150 nm (193 nm illumination, 0.93 NA) with less change for smaller pitches. Aerial image smearing did not significantly increase LER since it was directional. Simulation showed less than 1% reduction in NILS (compared to a static, smooth mask equivalent) with ideal alignment. In addition, description of pupil plane filtering on the transfer of mask roughness is given. When astigmatism-like aberrations were introduced in the pupil, transfer of mask roughness is decreased at best focus. It is important to exclude main diffraction orders from the filtering to prevent contrast and NILS loss. These ideas can be valuable as projection lithography approaches to conditions similar to IL (e.g. strong RET methods).

Extreme Ultraviolet Lithography (EUVL) at 13.5 nm is currently the most promising technology for advanced integrated circuit (IC) manufacturing nodes. Since the wavelength for EUVL is an order of magnitude smaller than current optical lithography systems (193 nm), wavelength scaled tolerances on lens manufacturing must be tightened to avoid image distortion and contrast loss as these scale with wavelength. Therefore understanding the aberrations of an EUVL system both in idle and production conditions is paramount. This study aims to assess a photoresist based technique for capturing pupil information of EUVL systems that can be implemented during full system use. Several data sets have been collected on an ASML EUV Alpha-Demo Tool (ADT) using the latest Center for Nanoscale Science and Engineering (CNSE) baseline resist Shin-Etsu SEVR139. Various one-dimensional and two-dimensional binary structures were imaged and used for pupil extraction in conjunction with computational modeling and simulations. Results show a stable extracted aberration signature over several measurements. Results also show that the method is sensitive to sub-nm levels of aberration change.

In this work, 3D mask modeling capabilities of Calibre will be used to assess mask topography impact on EUV imaging. The EUV mask absorber height and the non-telecentric illumination at mask level, modulate the captured intensity from the shadowed mask area through the reflective optics on to the wafer, named as the mask shadowing effect. On the other hand, thinning the mask absorber height results in unwanted background intensity, or called flare. A true compromise has to be taken into account for the height parameter of a EUV mask absorber. We will discuss the state-of-the-art 3D mask modeling capabilities, and will present methodologies to tackle the described EUV mask shadowing effect in Calibre software. The findings will be validated against experiments on ASML's NXE:3100 EUV scanner at imec. Masks with two different absorber heights will be evaluated on various combinations of features containing line/space and contact-hole.

In this paper, the progression of optical lithography from a spectral (or wavelength) perspective is presented. The interdependencies between materials, sources, design challenges, and scaling are described relative the needs of semiconductor device generations. Comparisons of materials and approaches for use in the blue-UV, the mid-UV, the deep-UV, and to EUV wavelengths are provided showing the spectral influences for choices made. Inorganic and organic lithography materials, sources of radiation, and incremental wavelength "shrink" are compared for lithography generations from g-line to EUV and beyond.

Extreme UV Lithography (EUVL) is generally accepted as the leading candidate for next generation lithography. Several challenges remain for EUVL, especially as its insertion point is pushed to finer resolution. Although diffractive scaling may suggest a transition to shorter EUVL wavelengths, several issues arise that would make that difficult. Challenges involve issues such as flare, multilayer (ML) bandwidth, and reflector throughput which tend to worsen with decreasing wavelength. In this study, we have evaluated the tradeoff between flare scaling effects and diffractive scaling effects for EUVL, where flare induced image degradation is likely to dominate as sub-13.5 nm wavelengths are considered. With surface scatter effects scaling as 1/λ2, the idea of longer wavelength (LW-EUVL) becomes interesting. Since a working wavelength is driven by the selection of ML materials (which are molybdenum and silicon for 13.5 nm), the identification of suitable alternatives is an initial challenge. We have optimized aluminum and various refractory metals at 17.2 nm and present results. The optimized combination of aluminum with yttrium, zirconium, and other metals result in theoretical reflectivity values above 75%. We also describe possibilities for alternative LW-EUVL sources for 17.2 nm operation as well as the impact on resist absorption, especially through halogens of higher molar absorption (such as fluorine). The impact on mask absorber materials is also presented, which may also exhibit increased absorbance, leading to a lowering of film thickness requirements.

As semiconductor lithography is pushed to smaller dimensions, process yields tend to suffer due to subwavelength topographical imaging effects. Three dimensional or "thick mask" effects result in such things as a pitch dependent best focus and, for alternating phase shift masks (AltPSMs), an intensity imbalance between etched and un-etched features. Corrective mask structures such as the dual trench AltPSM have been introduced to compensate for such intensity imbalances. In this work, the compensation of thick mask effects is explored using the manipulation of the pupil wavefront through the addition of spherical aberration. The wavefront has been experimentally varied through the manipulation of the lens aberration in a state of the art full field scanner. Results reveal that the influence of spherical aberration on best focus is predictable, allowing focus deviation through pitch to be tuned. Simulations further predict that aberration manipulation can provide compensation for thick mask effects by increasing the useable depth of focus for a particular set of features on both AltPSM and thicker film attenuated PSM masks. Such pupil wavefront correction has the potential to compensate for mask topography by matching thick mask effects to those of thin masks.

The unique properties of metamaterials, namely, their negative refractive index, permittivity, or permeability, have gained much recent attention. Research into these materials has led to the realization of a host of applications that may be useful to enhance optical nanolithography. A selection of materials has been examined both experimentally and theoretically to verify their support of surface plasmons, or lack thereof, in the DUV spectrum via the attenuated total reflection (ATR) method using the Kretschmann configuration. At DUV wavelengths, materials that were previously useful at mid-UV and longer wavelengths no longer act as metamaterials. Structured materials comprised of alternating layers of aluminum and aluminum oxide (Al2O3), as well as some other absorption-free dielectrics, exhibit metamaterial behavior, as do some elemental materials such as aluminum. These elemental and structured materials exhibit the best properties for use in plasmonic nanolithographic applications. Therefore, a simulator was created to examine material and thickness combinations to generate a tunable metamaterial for use in the DUV. A method for performing plasmonic interference lithography with this metamaterial has been proposed, with calculations showing the potential for half-pitch imaging resolution of 25 nm.

As semiconductor lithography is pushed to smaller dimensions, the process yields tend to suffer due to subwavelength imaging effects. In response, resolution enhancement technologies have been employed together with optimization techniques, specifically source mask optimization (SMO), which finely tunes the process by simultaneously optimizing the source shape and mask features. However, SMO has a limitation in that it fails to compensate for undesired phase effects. For mask features on the order of the wavelength, the topography of the mask can induce aberrations which bring asymmetry to the focus-exposure matrix (FEM) and ultimately decrease the process window. This paper examines the dependency of FEM asymmetry on factors such as the illumination coherency and lens induced spherical aberration. It is shown that lens induced primary spherical aberration strongly impacts the symmetry of the FEM. In this work, phase correction is achieved by incorporating the pupil plane in an optimization. It is shown that primary spherical aberration can correct for effects including the degraded depth of focus and the tilt in the FEM for a dual trench mask. A pupil function with an optimized coefficient of primary spherical aberration balances the spherical aberration induced by the mask topography.

Owing to its simplicity and ability to produce line/space gratings with the highest contrast, interferometric lithography is an ideal platform for developing novel double patterning materials and processes. However, lack of sub-10 nm alignment in most interferometric systems impedes its application. In this paper, litho-etch-litho double patterning on a two-beam interferometric system is achieved by converting Cartesian alignment into angular alignment. By concentrically rotating the wafer in the second exposure, the interleaved region between the two exposures allows for the evaluation of double patterning process and materials. Geometric analysis shows that angular alignment has greatly relaxed requirements compared to the Cartesian alignment. It is calculated that for 22 nm double patterning technologies, rotation angle larger than 0.12 degree is sufficient to produce 1 μm long frequency doubled line/space patterns with less than 10% CD variation.

The unique properties of metamaterials, namely their negative refractive index, permittivity, and permeability, have gained much recent attention. Research into these materials has led to the realization of a host of applications that may be useful to enhance optical nanolithography, such as a high pass pupil filter based on an induced transmission filter design, or an optical superlens. A large selection of materials has been examined both experimentally and theoretically through wavelength to verify their support of surface plasmons, or lack thereof, in the DUV spectrum via the attenuated total reflection (ATR) method using the Kretschmann configuration. At DUV wavelengths, materials that were previously useful at mid-UV and longer wavelengths no longer act as metamaterials. Composites bound between metallic aluminum and aluminum oxide (Al2O3) exhibit metamaterial behavior, as do other materials such as tin and indium. This provides for real opportunities to explore the potential of the use of such materials for image enhancement with easily obtainable materials at desirable lithographic wavelengths.

The three knobs of optical lithography, namely process factor k1, wavelength (λ) and numerical aperture (NA) have been constantly pushed to print smaller features. To get an equivalent k1 value below the fundamental limit of 0.25, double patterning (DP) has recently emerged as a viable solution for the 32nm lithography node. Various DP techniques exist such as litho-etch-litho-etch (LELE), litho-freeze-litho-freeze (LFLF) and self-aligned sidewall spacer. In this paper, the potential of higher order pitch division (pitch/N, N>2) for sub-32nm lithography is analyzed. Compared to double patterning, higher order pitch division lithography offers higher resolution but also faces significant challenges such as added cost and tighter process control. Several process schemes are proposed and compared in terms of complexity, susceptibility to alignment error and CD uniformity control. It is shown that the overlay budget does not necessarily decrease compared to double patterning. The main challenge in higher order pitch division comes from controlling the key processing steps that directly form lines or spaces. In addition, line CD control is easier than space (gap) control in all four "positive-tone" processes studied, similar to the double patterning case. Among the proposed processes, a freezing assisted double spacer (FADS) process that is simpler than the common sidewall spacer approach shows promise for balanced process control.

The challenge in obtaining good resist performance in terms of resolution, line width roughness and sensitivity at EUV wavelength forces to make more efficient use of photons that reach the wafer plan e than has been the case for traditional optical lithography. Theory demonstrates th at the current absorbance levels of EUV resists are quite far from optimal and absorbance should be increased. The most attractive pathway to achieve this is by increasing the fluorine content of EUV resists. The viability of this approach has been demonstrated using non-chemically amplified PMMA as model resist and comparing its photospeed with a fluorinated analogue. It has been demonstrated that the photospeed increases due to improved resist absorbance by ~1.5X, which is close to 1.7X that is predicted by the difference in absorbance. Further modeling studies support the experimental results and indicate an optimum for total film absorbance of ~0.20-0.25. Compared to cu rrent platforms this would corres pond to an increase in photospeed by ~1.7X which is accompanied with an improvement in LWR of ~1.14X. Combining this approach with the trends in EUV resists to increase PAG loading and include sensitizer in order to improve photospeed will likely provide a path for EUV resists th at will meet the specifications that are required for the 32nm and 22nm node. Keywords: EUV, absorbance, resist, interference lithography, simulation

The need to extend 193nm immersion lithography necessitates the development of a third generation (Gen-3) of high refractive index (RI) fluids that will enable approximately 1.7 numerical aperture (NA) imaging. A multipronged approach was taken to develop these materials. One approach investigated the highest-index organic thus far discovered. The second approach used a very high refractive index nanoparticle to make a nanocomposite fluid. This report will describe the chemistry of the best Gen-3 fluid candidates and the systematic approach to their identification and synthesis. Images obtained with the Gen-3 fluid candidates will also be presented for a NA≥ 1.7

The use of transverse electric (TE) polarization has dominated illumination schemes as selective polarization is used for high-NA patterning. The benefits of TE polarization are clear - the interference of diffracted beams remains absolute at oblique angles. Transverse magnetic (TM) polarization is usually considered less desirable as imaging modulation from interference at large angle falls off rapidly as the 1/cosθ. Significant potential remains, however, for the use of TM polarization at large angles when its reflection component is utilized. By controlling the resist/substrate interface reflectivity, high modulation for TM polarization can be maintained for angles up to 90° in the resist. This can potentially impact the design of illumination away from most recent TE-only schemes for oblique imaging angles (high NA). We demonstrate several cases of TM illumination combined with tuned substrate reflectivity for 0.93NA, 1.20NA, and 1.35NA and compare results to TE and unpolarized cases. The goal is to achieve a flat response through polarization at large imaging angles. An additional application of TM illumination is its potential use for double patterning. As double patterning and double exposure approaches are sought in order to meet the needs of 32nm device generations and beyond, materials and process engineering challenges become prohibitive. We have devised a method for frequency doubling in a single exposure using an unconventional means of polarization selection and by making use of the reflective component produced at the photoresist/substrate interface. In doing so, patterns can be deposited into a photoresist film with double density. As an example, using a projection system numerical aperture of 1.20, with water as an immersion fluid, and a conventional polyacrylate 193nm photoresist, pattern resolution at 20nm half-pitch are obtainable (which is 0.125lambda/NA). The process to transfer this geometry into a hardmask layer uses conventional materials, including the photoresist layer and thin film silicon oxide based materials.

Hyper-NA immersion lithography is becoming the dominant technology for semiconductor device fabrication. At the corresponding large angles in the image media, control of polarization becomes necessary. Innovative approaches to polarization control are evolving to provide maximum enhancement from polarization effects.

As immersion lithography is extended to ever increasing resolution, the resulting propagation angles in the materials involved become closer to grazing than to normal incidence. Classical laws of refraction and reflection cannot be used with either assumption however, as a collection of angles may exist across the entire range. Fresnel reflection at these angles becomes large enough that small disparities in refractive indices at material interfaces may lead to adverse effects. As an example, when water is used at numerical apertures approaching its refractive index, reflection effects are greater than the constraints imposed by refraction or absorption. This will limit the maximum NA value allowed by any given material to values sufficiently lower than its refractive index. Additionally, we have grown accustom to expanding the application of the Snell-Descartes Law to materials with low absorption, assuming that the contribution of the imaginary component of the refractive index is negligible. This is not the case for photoresists, fluids, or glasses, which can not strictly be considered as non-absorbing media. We have expanded the Snell-Descartes Law for absorbing media, with some interesting consequences. We will show that there is no real limit on the numerical aperture into a material, so long as its extinction coefficient is not zero. The relationship that lithographers have been using recently where NA< min[n glass , n fluid , n resist ] will be shown to be inadequate and imaging at numerical apertures up to 1.85 will be demonstrated using materials with significantly lower (real) refractive index values.

The progress of optical lithography has approached the sub-30 nm regime using 193nm excimer lasers as the exposure sources. To increase the numerical aperture (NA) further, many issues, especially those related to materials, need to be addressed. In this paper, we present the analytical and experimental results of oblique two-beam lithography with sapphire (Al 2 O 3 ) as the optical material. At 193nm, the index of sapphire is 1.92 while the typical index of a photoresist is near 1.70. Classical theory predicts that, ignoring the absorbance in the photoresist, once the NA is greater than the photoresist refractive index, no energy will be transmitted across the sapphire/photoresist boundary due to total internal reflection. However, it can be shown that the absorbance in the resist prevents a "critical angle" and total internal reflection will not occur. Photoresist exposure can result even when NA is greater than the photoresist refractive index. The image profile is strongly affected by the real and imaginary parts of the photoresist refractive index. Optimization of photoresist optical properties is necessary for good image profile. Lutetium aluminum garnet (Lu 3 Al 5 O 12 or LuAG with an index 2.14 at 193 nm) is also investigated as an alternative lens material.

State of the art lithography is continually driven to resolve increasingly smaller features, forcing k 1 values for lithography processes ever lower. In order to image these difficult features with reliable fidelity, lithographers must increasingly use Resolution Enhancement Techniques (RETs). One such technique that is proposed in this paper uses small, sub-wavelength grooves placed in close proximity to an aperture. These sub-wavelength grooves create evanescent fields bound to the surface between the absorber and the mask substrate, decaying exponentially in lateral directions. In this work we demonstrate the ability to use such Evanescent Wave Assist Features (EWAFs) to enhance the propagating near and far field energy within openings such as slits and contacts. Using a Finite Difference Time Domain model, the effects of these evanescent wave assist features are explored in both the near and far field regions. Several cases of absorber material, feature type, spacing, and illumination will be presented.

Double patterning has been proposed as a method to extend DUV lithography to 32nm and below. Here, a new form of double, or higher, multiple exposure technique is proposed. This new form of lithography uses a combination of Quantum State Control (QuSC) chemistry, Amplitude Modulation Optical Lithography (AMOL), and multiple micro-stepped exposures, without development between exposures. Further it is proposed to use this form of lithography (called QuSC-litho), to pattern a perfect grating grid, and to trim this grid with an earlier generation lithography tool. QuSC lithography uses short optical pulses to modulate a photochemical pathway while an intermediate is still in a defined vibrational excited state. This is a variation of Stimulated Emission Depletion Microscopy (STED) developed for fluorescence microscopy. With this approach immersion tools that produce 90 nm pitch and 45 nm features should be able to pattern levels with 22 nm features with a 1:1 line-space ratio. This approach is much less sensitive to misalignment than present double patterning approaches. Key to successful deployment of QuSC lithography is defining a resist photochemistry consistent with the QuSC process. There are several approaches to Photo Acid Generator (PAG) - matrix interaction that may be consistent with this approach.

In-situ aberration measurement often requires indirect methods that retrieve the pupil phase from the measured images and presents unique challenges to the engineers involved. Phase wheel monitor allows such in-situ measurement of aberrations in photolithography systems. The projection lens aberrations may be obtained with high accuracy from images of phase wheel targets printed in photoresist. As a result, the photolithography tool optics can be characterized under standard wafer printing conditions. Resulting features are mathematically analyzed to extract information about the aberrations in optics. We use a detection algorithm and multi-domain modeling to process resist images and determine the image deviation from the ideal shape, which in turn allow the amount of aberration introduced by the optical system to be quantified. Experimental results are shown and multiple measurements on the same tool before and after system corrections are compared.

In a search for alkane candidates for 193 nm immersion fluids, several alkanes and cycloalkanes were synthesized, purified and screened to ascertain their absorption at 193 nm, refractive index and temperature dispersion coefficient in the context of the actual application. In general, cycloalkanes, and more specifically polycycloalkanes, possess a higher refractive index than do linear alkanes. Decalin, cyclodecane, perhydrophenanthreme (PHP), perhydrofluorene (PHF) and perhydropyrene (PHPY) are examined as potential second and third generation immersion fluids. The use of perhydropyrene, which possesses a high refractive index of 1.7014 at 193 nm, may be limited as an immersion fluid because of high absorption at 193 nm. Mixtures of cycloalkanes can lead to a higher enhancement of the refractive index together with a decrease on the viscosity. Exhaustive purification of the fluids is a critical step in determining the real absorption of the different fluids at 193 nm. Two simple purification processes of these cycloalkanes were developed that led to low absorption fluids in the VUV region. The possibility of forming the oxygen complex in aerated fluids was reduced by purging samples with argon or nitrogen. This easy elimination of the oxygen complex shows the weak bonding nature of this complex.

New applications of evanescent imaging for microlithography are introduced. The use of evanescent wave lithography (EWL) has been employed for 26nm resolution at 1.85NA using a 193nm ArF excimer laser wavelength to record images in a photoresist with a refractive index of 1.71. Additionally, a photomask enhancement effect is described using evanescent wave assist features (EWAF) to take advantage of the coupling of the evanescent energy bound at the substrate-absorber surface, enhancing the transmission of a mask opening through coupled interference.

An automated aberration extraction method is presented which allows extraction of lithographic projection lens' aberration signature having only access to object (mask) and image (wafer) planes. Using phase-wheel targets on a two-level 0/ir phase shift mask, images with high sensitivity to aberrations are produced. Zemike aberration coefficients up to 9 th order have been extracted by inspection of photoresist images captured via top-down SEM. The automated measurement procedure solves a multi-dimensional optimization problem using numerical methods and demonstrates improved accuracy and minimal cross-correlation. Starting with a detailed procedure analysis, recent experimental results for 193-nm projection optics in commercial full field exposure tools are discussed with an emphasis on the performance of the aberration measurement approach.

In a photolithographic system, the mask patterns are imaged through a set of lenses on a resist-coated wafer. The image of mask patterns physically can be viewed as the interference of the plane waves of the diffraction spectrum captured by the lens set incident on the wafer plane at a spectrum of angles. Two-beam interference fringe is the simplest format of the image. Consequently, two-beam interferometric lithography is often employed for photolithographic researches. For two-beam interferometric lithography, beam pointing instability of the illumination source can induce fringe displacement, which results in a loss of fringe contrast if it happens during the exposure. Since some extent of beam pointing instability is not avoidable, it is necessary to investigate its effects on the contrast of the interference fringe. In this paper, the effects of beam pointing instability associated with a two-beam interferometric lithography setup are analyzed. Using geometrical ray tracing technique and basic interference theory, the relationship between the beam tilt angle and interference fringe displacement is established. For a beam pointing instability with random distribution, the resulted fringe contrast is directly proportional to the Fourier transform of the pointing distribution evaluated at 1/(2π). The effect of a pointing instability with normal distribution on interference contrast is numerically investigated.

Immersion interferometric lithography has been applied successfully to semiconductor device applications, but its potential is not limited to this application only. This paper explores this imaging technology for the production of three-dimensional nano-structures using a 193 nm excimer laser and immersion Talbot interferometric lithographic tool. The fabrication of 3-D photonic crystals for the UV spectrum is still considered to be a challenge. A systematic analysis of immersion lithography for 3-D photonic crystal fabrication will be provided in this paper. Significant progress has been made on optical immersion lithography since it was first proposed. Two-beam immersion interferometric lithography can provide sub-30nm resolution. By changing the exposure parameters, such as the numerical aperture of the exposure system, the polarization states and wavelength of the illumination source, 30 nm polymeric nanospheres with different crystal structures can be fabricated.

An approach to measurement of resist CD response to image modulation and dose is presented. An empirical model with just three terms is used to describe this response, allowing for direct calculation of photoresist modulation curves. A thresholded latent image response model has been tested to describe CD response for both 90 nm and 45 nm geometry. An assumption of a linear optical image to photoresist latent image correlation is shown as adequate for the 90 nm case, while the 45 nm case demonstrates significant non-linear behavior. This failure indicates the inadequacy of a "resist blur" as a complete descriptive function and uncovers the need for an additional spread function in OPE-style resist models.

Projection and interference imaging modalities for application to IC microlithography were compared at the 90 nm imaging node. The basis for comparison included simulated two-dimensional image in resist, simulated resist linesize, as well as experimental resist linesize response through a wide range of dose and focus values. Using resist CD as the main response (both in simulation and experimental comparisons), the two imaging modes were found nearly equivalent, as long as a suitable Focus-Modulation conversion is used. A Focus-Modulation lookup table was generated for the 45 nm imaging node, and experimental resist response was measured using an interferometric tool. A process window was constructed to match a hypothetical projection tool, with an estimated error of prediction of 0.6 nm. A demodulated interferometric imaging technique was determined to be a viable method for experimental measurement of process window data. As long as accurate assumptions can be made about the optical performance of such projection tools, the response of photoresist to the delivered image can be studied experimentally using the demodulated interferometric imaging approach.