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The microtag concept is an anticounterfeiting and security measure. Microtags are computer-generated holograms (CGH's) consisting of 150-nm lines arranged to form 300-nm-period gratings. The microtags that we describe were designed for readout at 442nm . The smallest microtag measures 56micromx80 microm when viewed at normal incidence. The CGH design process uses a modified iterative Fourier-transform algorithm to create either phase-only or phase-and-amplitude microtags. We also report on a simple and compact readout system for recording the diffraction pattern formed by a microtag. The measured diffraction patterns agree very well with predictions.

While interferometry is routinely used for the characterization and alignment of lithographic optics, the ultimate performance metric for these optics is printing in photoresist. The comparison of lithographic imaging with that predicted from wavefront performance is also useful for verifying and improving the predictive power of wavefront metrology. To address these issues, static, small-field printing capabilities have been added to the EUV phase-shifting point diffraction interferometer (PS/PDI) implemented at the Advanced Light Source at Lawrence Berkeley National Laboratory. The combined system remains extremely flexible in that switching between interferometry and imaging modes can be accomplished in approximately two weeks. Relevant printing studies with the ETS projection optics require illumination partial coherence with a of approximately 0.7. However, this a value is very different from the coherent illumination naturally provided by synchrotron undulator beamline and required by the high accuracy EUV PS/PDI. Adding printing capabilities to the PS/PDI experimental system has thus necessitated the development of an alternative illumination system capable of quantitatively reducing the inherent coherence of the beamline. The implemented illuminator is an angular scanning system capable of in situ coherence control. Moreover, this illuminator design readily enables the implementation of conventional resolution-enhancing pupil fills and modeling of the unique Engineering Test Stand (ETS) pupil fill. This new static microfield exposure tool has been used to lithographically characterize the static imaging performance of the ETS Set-2 projection optics. Excellent performance has been demonstrated down to the 70-nm 1:1 line/space level with a focus latitude exceeding 1 μm and a dose latitude of approximately 8%. Moreover, dense-line printing down to a resolution of 50 nm has been demonstrated.

The Engineering Test Stand (ETS) is an EUV lithography tool designed to demonstrate full-field EUV imaging and provide data required to accelerate production-tool development. Early lithographic results and progress on continuing functional upgrades are presented and discussed. In the ETS a source of 13.4 nm radiation is provided by a laser plasma source in which a Nd:YAG laser beam is focused onto a xenon- cluster target. A condenser system, comprised of multilayer-coated and grazing incidence mirrors, collects the EUV radiation and directs it onto a reflecting reticle. The resulting EUV illumination at the reticle and pupil has been measured and meets requirements for acquisition of first images. Tool setup experiments have been completed using a developmental projection system with (lambda) /14 wavefront error (WFE), while the assembly and alignment of the final projection system with (lambda) /24 WFE progresses in parallel. These experiments included identification of best focus at the central field point and characterization of imaging performance in static imaging mode. A small amount of astigmatism was observed and corrected in situ, as is routinely done in advanced optical lithographic tools. Pitch and roll corrections were made to achieve focus throughout the arc-shaped field of view. Scan parameters were identified by printing dense features with varying amounts of magnification and skew correction. Through-focus scanned imaging results, showing 100 nm isolated and dense features, will be presented. Phase 2 implementation goals for the ETS will also be discussed.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The Engineering Test Stand (ETS) is a developmental lithography tool designed to demonstrate full-field EUV imaging and provide data for commercial-tool development. In the first phase of integration, currently in progress, the ETS is configured using a developmental projection system, while fabrication of an improved projection system proceeds in parallel. The optics in the second projection system have been fabricated to tighter specifications for improved resolution and reduced flare. The projection system is a 4-mirror, 4x-reduction, ring-field design having a numeral aperture of 0.1, which supports 70 nm resolution at a k{sub 1} of 0.52. The illuminator produces 13.4 nm radiation from a laser-produced plasma, directs the radiation onto an arc-shaped field of view, and provides an effective fill factor at the pupil plane of 0.7. The ETS is designed for full-field images in step-and-scan mode using vacuum-compatible, magnetically levitated, scanning stages. This paper describes system performance observed during the first phase of integration, including static resist images of 100 nm isolated and dense features.

In Extreme Ultraviolet (EUV) lithography, sub-resolution reticle substrate defects which are overcoated during the multilayer coating process could introduce proximity phase errors. A strategy for mitigating this problem is to coat the substrate with a smoothing layer prior to ML deposition. A spherical defect will be reduced to a low aspect Gaussian bump. In order to understand the smoothing requirements necessary to render a defect non-critical, we have utilized a simplified 3-D lithographic modeling approach to study the effect of a Gaussian bump in proximity to both 70 nm 1:3 L/S and 35 nm 1:3 L/S printed with a 0.1 and 0.25 numerical aperture system, respectively. The results quantify that the smoothing approach can be successfully used to render a defect non-critical by two means: 1) reducing the defect volume or 2) by reducing the slope of the Gaussian bump.

The stringent requirements placed on sub-100 nm feature lithography require the development of a low distortion mask. The distortions due to the fabrication process of extreme ultraviolet lithography masks have been predicted using finite element methods. The investigation included the simulation of the mask blank fabrication, pattern transfer and mounting. Film stress gradient effects for the multilayer stack, absorber layer and resist layer were also evaluated, considering both in-plane and out-of-plane distortions. To assist in optimizing the final substrate and pattern dimensions, a number of different substrate formats and materials were considered. To determine the effects of pattern density, different types of patterns were modeled, including uniform coverage, a 'half-and-half' pattern and an orthotropic feature pattern. Reticle placement errors were less than 2 nm at the mask level (0.5 nm at the wafer level) for nearly all cases studied.

Lithographic masks must maintain dimensional stability during exposure in a lithographic tool to minimize subsequent overlay errors. In extreme ultraviolet lithography (EUVL), multilayer coatings are deposited on a mask substrate to make the mask surface reflective at EUV wavelengths. About 40% of the incident EUV light is absorbed by the multilayer coating which leads to a temperature rise. The choice of mask substrate material and absorber affects the magnitude of thermal distortion. Finite element modeling has been used to investigate potential mask materials and to explore the efficiency of various thermal management strategies. An experimental program was conducted to validate the thermal models used to predict the performance of EUV reticles. The experiments closely resembled actual conditions expected within the EUV tool. A reticle instrumented with temperature sensors was mounted on a scanning stage with an electrostatic chuck. An actively cooled isolation plate was mounted in front of the reticle for thermal management. Experimental power levels at the reticle corresponding to production throughput levels were utilized in the experiments. Both silicon and low expansion glass reticles were tested. Temperatures were measured a several locations on the reticle and tracked over time as the illuminated reticle was scanned. The experimental results coupled with the predictive modeling capability validates that the assertion that the use of a low expansion glass will satisfy image placement error requirements down to the 30 nm lithographic node.

The thickness of the photoresist directly impacts the etch stand off and may impact the number of defects in the spin- coated film. However, the maximum imaging layer thickness for extreme ultraviolet lithography (EUVL) is limited by absorption of the radiation. Attenuation in photoresist materials at relevant EUV wavelengths was calculated with atomic extinction coefficients provided from Henke et al. The calculations indicated that photoresist materials have an optical density (O.D.) of 4.0 micrometer-1 (base e) so that 100 nm thick imaging layers have approximately 67% transmission at 13.4 nm wavelength. Using Prolith/3DTM (Finle Technologies, Austin, TX) simulations of the effect of highly attenuating materials on sidewall slope were done and shown to be small. Imaging experiments were performed in a commercially-available DUV resist material on the 10 X II microstepper and with an improved EUV resist formulation. The imaging results agreed well with the calculations. Top down and cross-section images showed good sidewall profiles in 95 nm thick films at the nominal dose because over 68% of the energy was transmitted through the film. When the thickness of the film was increased, the dose was increased slightly to compensate for the absorption while good sidewall profiles and linearity were maintained. Photoresist thicknesses as high as 145 nm were imaged with a 35% increase in dose. Results are also shown for a single layer resist exposed at 175 nm thickness with only slight sidewall degradation. It is shown that the imaging layer thickness for 13.4 nm lithography is likely to be 120 +/- 15 nm. If 11.4 nm wavelength radiation is chosen for EUV lithography, it is shown that thicknesses of 170 nm is possible.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We propose the use of optical proximity correction on absorber features to compensate for the effect of sub-resolution multilayer defects that would otherwise induce a critical error in linewidth. A series of defect printability and compensation experiments utilizing programmed multilayer defects are presented which demonstrate this approach. The amount of absorber removal for defect compensation depends on system imaging performance and the quality of the absorber removal process. A process flow for the mask fabrication, defect characterization and compensation is presented.

We report on the comparison of defect printability experimental results with at-wavelength defect inspection and printability modeling at extreme ultraviolet (EUV) wavelengths. Two sets of EUV masks were fabricated with nm- scale substrate defect topographies patterned using a sacrificial layer and dry-etch process, while the absorber pattern was defined using a subtractive metal process. One set of masks employed a silicon dioxide film to produce the programmed defects, whereas the other set used chromium films. Line-, proximity- and point-defects were patterned and had lateral dimensions in the range of 0.2 micrometer X 0.2 micrometer to 8.0 micrometer X 1.5 micrometer on the EUV reticle, and a topography in the range of 8 nm - 45 nm. Substrate defect topographies were measured by atomic force microscopy (AFM) before and after deposition of EUV-reflective Mo/Si multilayers. The programmed defect masks were then characterized using an actinic inspection tool. All EUVL printing experiments were performed using Sandia's 10x- reduction EUV Microstepper, which has a projection optics system with a wavefront error less than 1 nm, and a numerical aperture of 0.088. Defect dimensions and exposure conditions were entered into a defect printability model. In this investigation, we compare the simulation predictions with experimental results.

Calculations were carried out to study the effectiveness of a gas curtain for mitigating contamination from hydrocarbon gasesin extreme ultraviolet (EUV) lithography tools. The momentum, energy and mass transport in the curtain and surroundingregions were calculated using a two dimensional finite volume model of a lithography tool. Argon was used for the gascurtain. The results from the calculations quantify the relationship between the argon flow rate and the hydrocarbon partialpressure in the region of the optical elements. In general, the hydrocarbon partial pressure decreases as the mass transfer Peclet number, Pe VhJD, increases, where V is the velocity of the argon, h is the thickness of the gas curtain and D is themass diffusivity. An argon flow rate of 12.2 torr•l/s is required to reduce the hydrocarbon partial pressure from iO to l0b0ton in the specific lithography tool considered here. The EUV transmission in the tool with the gas curtain is 0.93. This is alarge improvement over the 0.5

Lithographic masks must maintain dimensional stability during exposure in a wafer stepper. In extreme UV lithography, multilayer coatings are deposited on a flat mask, substrate to make the mask surface reflective at EUV wavelengths. About 40 percent of the incident EUV radiation is absorbed by the multilayer coatings causing a temperature rise. The choice of mask substrate material affects dimensional stability due to thermal expansion and/or deformation. Finite element modeling has ben used to investigate the proper choice of mask substrate material and to explore the efficacy of various thermal management strategies. This modeling indicates that significant machine design and engineering challenges are necessary in order to employ Si as a mask substrate. Even if these challenges can be met, the thermal expansion of Si is likely to be too large to meet overlay error budgets for lithography at ground rules beyond the 100 nm technology node. ULE - a single phase, fused silica glass doped with titania - has near zero thermal expansion at the temperatures where EUV lithography is performed. Due to its small coefficient of thermal expansion, ULE does not undergo appreciable instantaneous or transient thermal expansion that results in image placement error.

Results of numerical studies on the deformation of multilayer coated optics for EUV lithography are presented and compared. The primary materials-dependent characteristics of EUV multilayer mirrors are reflectance, stress, and stability. Understanding and controlling these mirror characteristics is critical for the efficient operation of an EUVL system. Optimized for reflectance, the multilayer coatings typically have film stress values large enough to deform the optic. We use finite element models to predict the distortion caused by the film stress. Input parameters to the models include the size, shape, and material properties of the optical substrate as well as the magnitude and spatial variations of the film stress. The distortion results are coupled with optical analysis software to predict wavefront aberrations for the candidate optical design. Finally, lithographic analysis software is used to predict the aerial image and linewidth variation. Model predictions have guided the design of the optical substrates so that lithographic errors induced by film- stress-deformation are expected to be within the acceptable error budget.

Extreme Ultraviolet Lithgraphy (EUVL) is an emerging technology for fabrication of sub-100 nm feature sizes on silicon, following the SIA roadmap well into the 21st century. The specific EUVL system described is a scanned, projection lithography system with a 4:1 reduction, using a laser plasma EUV source. The mask and all of the system optics are reflective, multilayer mirrors which function in the extreme ultraviolet at 13.4 nm wavelength. Since the masks are imaged to the wafer exposure plane, mask defects greater than 80% of the exposure plane CD (for 4:1 reduction) will in many cases render the mask useless, whereas intervening optics can have defects which are not a printing problem. For the 100 nm node, we must reduce defects to less than 0.01/cm² @ 80nm or larger to obtain acceptable mask production yields. We have succeeded in reducing the defects to less than 0.1/cm² for defects larger than 130 nm detected by visible light inspection tools, however our program goal is to achieve 0.01/cm² in the near future. More importantly though, we plan to have a detailed understanding of defect origination and the effect on multilayer growth in order to mitigate defects below the 10-2/cm² level on themore » next generation of mask blank deposition systems. In this paper we will discuss issues and results from the ion-beam multilayer deposition tool, details of the defect detection and characterization facility, and progress on defect printability modeling.« less

The capabilities of the EUV 10x microstepper have been substantially improved over the past year. The key enhancement was the development of a new projection optics system with reduced wavefront error, reduced flare, and increased numerical aperture. These optics and concomitant developments in EUV reticles and photoresists have enabled dramatic improvements in EUV imaging, illustrated by resolution of 70 nm dense lines and spaces (L/S). CD linearity has been demonstrated for dense L/S over the range 100 nm to 80 nm, both for the imaging layer and for subsequent pattern transfer. For a +/- 10 percent CD specification, we have demonstrated a process latitude of +/- micrometers depth of focus and 10 percent dose range for dense 100 nm L/S.

Extreme UV Lithography (EUVL) is one of the leading candidates for the next generation lithography, which will decrease critical feature size to below 100 nm within 5 years. EUVL uses 10-14 nm light as envisioned by the EUV Limited Liability Company, a consortium formed by Intel and supported by Motorola and AMD to perform R and D work at three national laboratories. Much work has already taken place, with the first prototypical cameras operational at 13.4 nm using low energy laser plasma EUV light sources to investigate issues including the source, camera, electro- mechanical and system issues, photoresists, and of course the masks. EUV lithograph masks are fundamentally different than conventional photolithographic masks as they are reflective instead of transmissive. EUV light at 13.4 nm is rapidly absorbed by most materials, thus all light transmission within the EUVL system from source to silicon wafer, including EUV reflected from the mask, is performed by multilayer mirrors in vacuum.

Extreme ultraviolet lithography (EUVL) is a candidate technology for the microelectronics industry with design rules for 0.1 {micro}m features and beyond. When characterizing an extreme ultraviolet (EUV) lithographic optical system, visible light interferometry is limited to measuring wavefront aberration caused by surface figure error while failing to measure wavefront errors induced by the multilayer coatings. This fact has generated interest in developing interferometry at an EUV camera`s operational wavelength (at-wavelength testing), which is typically around 13 nm. While a laser plasma source (LPS) is being developed as a lithography production source, it has generally been considered that only an undulator located at a synchrotron facility can provide the necessary laser-like point source for EUV interferometry. Although an undulator-based approach has been successfully demonstrated, it would be advantageous to test a camera in its operational configuration. The authors are developing the latter approach by utilizing extended source size schemes to provide usable flux throughput. A slit or a grating mounted in front of the source can provide the necessary spatial coherence for Ronchi interferometry. The usable source size is limited only by the well-corrected field of view of the camera under test. The development of this interferometer will be presented.

Thermal and structural effects will be an important consideration for all advanced lithography approaches targeting the 100nm technology generation and beyond. Such effects can contribute to loss of CD control, decrease in process latitude and reticle-wafer overlay error. This necessitates a system approach to account for thermo- mechanical effects in a complete system performance analysis of an EUV lithography tool. Multilayer-coated mirrors will typically absorb 35-40 percent of the in-band radiation causing thermal deformation of the mirror figure. In addition, Mo-Si multilayer films are deposited with compressive stress of approximately 350 MPa, which will also serve to deform the mirror substrate. To study these effects, we have inter-connected the capabilities of several software packages which include thermal and structural finite element, optical, and lithographic analysis. This enables us to determine the impact of mechanical effects on lithographic metrics such as the exposure-defocus process window, pattern placement and throughput. This paper includes result from a theoretical study of an EUV alpha tool with a wafer throughput of 20 200 mm wafers per hour for the 100nm technology generation.

The Sandia EUV 10x microstepper system is the result of an evolutionary development process, starting with a simple 20x system, progressing through an earlier 10x system, to the current system that has full microstepper capabilities. The 10x microstepper prints 400-micrometers -diameter fields at sub- 0.10-micrometers resolution. Upgrades include the replacement of the copper wire target with a pulsed xenon jet target, construction of an improved projection optics system, the addition of a dose monitor a d an aerial image monitor, and the addition of a graphical user interface to the system operation software. This paper provides an up-to-date report on the status of the microstepper.

We have developed a method for encoding phase and amplitude in microscopic computer-generated holograms (microtags) for security applications. An 8 X 8-cell phase-only and an 8 X 8-cell phase-and-amplitude microtag design have been fabricated in photoresist using the extreme-ultraviolet (13.4 nm) lithography tool developed at Sandia National Laboratories. Each microtag measures 80 by 160 microns and contains features 0.2 micrometers wide. Fraunhofer-zone diffraction patterns can be obtained from fabricated microtags without any intervening optics and compare very favorably with predicted diffraction patterns. In this paper, we present the results of a preliminary rigorous coupled-wave analysis of microtags. Microtags are modeled as sub-wavelength gratings of a trapezoidal profile. Only TE polarization is modeled. The analysis in this paper is concerned with the determination of optimal microtag design parameter values and tolerances on those parameters. The parameters are wall-slope angle, grating duty cycle, grating depth, and metal-coating thickness. Our findings indicate that diffraction-efficiency monotonically increases as the gratings are: (1) deepened and (2) coated with metal. Coating with metal achieves a more significant improvement and is easier to implement. The tolerance on the wall slope angle is very loose. The optimal grating duty cycle is between 0.5 and 0.6, depending on the presence of the metallic coating. The application of a protective coating on metal-coated microtags leads to an increase in diffraction efficiency and represents a practical configuration for these elements.

A laboratory extreme ultraviolet lithography tool (EUVL) has been assembled atSandia National Laboratories [1]. Its major components include a Schwarzschildcamera with 0.1 jim resolution integrated with a laser plasma source (LPS) and amagnetically levitated stage. Other subsystems are a grazing-incidence optical system to maintain focus, and a through-the-lens Moire alignment system for overlay. This exposure tool is being used to study integration issues for EUVL.Experiments have been performed to characterize the alignment system's performance. The measured sensitivity of the alignment system is 15 nm 3. 1. INTRODUCTION A laboratory extreme ultraviolet lithography tool (EUVL) has been assembled atSandia National Laboratories' . Its major components include a Schwarzschild camera with 0.1 jim resolution integrated with a laser plasma source (LPS) and a magnetically levitated stage. Other subsystems include a grazing-incidence opticalsystem to maintain focus and an alignment system for overlay. This exposure toolis being used to study integration issues for EUVL. In order to demonstrate thesuccessful integration of the system, microelectronic devices and circuits will befabricated with at least one lithography step performed with EUVL.In this paper, we will describe in detail the EUVL tool and its capabilities. We willalso report results from recent experiments performed to characterize theperformance of the visible through-the-lens (TTL) Moire alignment system.

Microtags are microscopic computer-generated holograms with 130-nm features and are mass- producible with EUVL. This fabrication method renders microtags difficult to counterfeit. Applications include tagging and tracking of microprocessors, memory chips, currency, and credit cards.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The microtag concept has been developed for security purposes. Microtags can be mass-produced using an advanced extreme-ultraviolet lithography (EUVL) tool currently under development at Sandia National Laboratories in Livermore. This method of fabrication results in features small enough (down to 100 nm) to deter most attempts at counterfeiting. Microtags may therefore be used to protect integrated circuits, currency, credit cards, and smart cards.

When characterizing an extreme ultraviolet (EUV) lithographic optical system, visible light interferometry is limited to measuring wavefront aberration caused by surface figure error while failing to measure wavefront errors induced by the multilayer coatings. This fact has generated interest in developing interferometry at an EUV camera's operational wavelength (at wavelength testing), which is typically around 13 nm. While a laser plasma source (LPS) is being developed as a lithography production source, it has generally been considered that only an undulator located at a synchrotron facility can provide the necessary laser-like point source for EUV interferometry. Although an undulator-based approach has been successfully demonstrated, it would be advantageous to test a camera in its operational configuration. We are developing the latter approach by utilizing extended source size schemes to provide usable flux throughput. A slit or a grating mounted in front of the source can provide the necessary spatial coherence for Ronchi interferometry. The usable source size is limited only by the well-corrected field of view of the camera under test. The developemnt of this interferometer will be presented.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The demand for smaller critical dimensions in advanced computer chips has driven projection lithography to shorter wavelengths. Deep ultraviolet systems, operating at 248 nm, are commercially available and 193 nm lithography is under development. This paper describes a research and development program to extend this trend to extreme ultraviolet (EUV) wavelengths, in the range of 11 nm to 14 nm. Research and development activities are concentrated in four major areas: (1) aspheric reflectors required in an advanced EUV camera, (2) surface- imaging resists having the required resolution and sensitivity, (3) defect-free reflecting substrates required for mask fabrication, and (4) a compact source of EUV radiation. Each of these issues is discussed and efforts to address them are described. An intermediate EUVL laboratory tool, currently under construction, will also be described. This system uses a 10x reduction Schwarzschild camera and a magnetically levitated wafer stage driven by a digital feedback controller. This system represents the first attempt at integrating all major subsystems into an EUVL laboratory tool, suitable for use in device fabrication experiments.

This paper reports results from a study of defect printability for extreme ultraviolet lithographic masks (EUVL). Imaging experiments were preformed with an EUVL mask containing programmed defects. The mask contained defects of different sizes and locations with respect to the absorber patterns. Since an EUVL mask consists of absorber layer patterned above a multilayer-coated substrate, both substrate defects, located below the reflective coating, and absorber defects, located above the reflective coating, were included on the programmed defect mask. The absorber layer was patterned using a process previously described by Tennant et al. Imaging was performed using a 10X Schwarzschild camera operating at 13.4 nm with a numerical aperture of 0.08, corresponding to a Rayleigh resolution of 0.1 micrometers . This system has an effective exposure field of 0.4 mm diameter. Both positive-tone and negative-tone resists were used. Measurements of the defect-induced linewidth variations on the printed resist lines were performed with scanning electron microscopy (SEM) and atomic force microscopy (AFM). Results show that substrate defects are more printable compared to defects of the same sizes located above the reflective coating. In addition, defects located in the center of a clear region in lines-and- space pattern are more printable compared to those located nearer to the absorber lines.

The development of a laboratory EUV lithography tool based on a laser plasma source, a 10x Schwarzschild camera, and a magnetically levitated wafer stage is presented. Interferometric measurements of the camera aberrations are incorporated into physical-optics simulations to estimate the EUV imaging performance of the camera. Experimental results demonstrate the successful matching of five multilayer reflecting surfaces, coated to specification for a wide range of figure and incidence angle requirements. High-resolution, 10x-reduction images of a reflection mask are shown.

The scanning soft x-ray photoemission microscope (MAXIMUM) operating at the Synchrotron Radiation Center at the University of Wisconsin-Madison has been substantially upgraded. The major upgrades are: installation of a new beam line that is optimized for the microscope; new optical mount and alignment system for the Schwarzschild objective; a new scanning stage; installation of a cylindrical mirror analyzer; implementation of an ultrahigh vacuum sample preparation chamber and transfer system; a new window-driven data acquisition program that is more flexible and user-friendly. The new system had demonstrated better than 0.1 micrometers spatial resolution, and photoemission data with 350 meV energy resolution has been obtained.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The extreme ultraviolet (EUV) Engineering Test Stand (ETS) is a step-and-scan lithography tool that operates at a wavelength of 13.4 nm. It has been developed to demonstrate full-field EUV imaging and acquire system learning for equipment manufacturers to develop commercial tools. The initial integration of the tool is being carried out using a developmental set of projection optics, while a second, higher-quality, projection optics is being assembled and characterized in a parallel effort. We present here the first lithographic results from the ETS, which include both static and scanned resist images of 100 nm dense and isolated features throughout the ring field of the projection optics. Accurate lithographic models have been developed and compared with the experimental results.

Improvements in mask blank yield by the reduction in defects dramatically reduces the reticle cost of any lithography. Extreme ultraviolet lithography (EUVL) masks typically consist of a substrate coated with reflective multilayer (ML) materials (e.g., Mo and Si), followed by a sacrificial or “repair” layer (e.g., SiO2) and top absorber layer material (e.g., Cr or TaSiNx). Defects of the following two types exist: substrate and absorber defects. Substrate defects are generally below the absorber and buffer layers, i.e., at the substrate–ML interface or within the ML stack, whereas the absorber defects are located at the absorber layer. In addition, the printability of substrate-type defects may be reduced by coating the defects with a planarizing or smoothing layer. In this work, we discuss simulation and experimental results that compare the printability of programmed defect reticles with and without smoothing layers covering the programmed defect. We propose several criteria for smoothing layer specific...

We propose the use of optical proximity correction on absorber features to compensate for the effect of subresolution multilayer defects that would otherwise induce a critical error in linewidth. Initial experiments have been performed which validate this concept. Process window simulations quantify the practical limits of this technique.

Numerical simulation is used in the development of an extreme ultraviolet lithography Engineering Test Stand. Extensive modeling was applied to predict the impact of thermal loads on key lithographic parameters such as image placement error, focal shift, and loss of CD control. We show that thermal issues can be effectively managed to ensure that their impact on lithographic performance is maintained within design error budgets.

Thermal deformation of reticles will likely become an important consideration for all advanced lithography techniques targeting 130 nm features and below. Such effects can contribute to image placement errors and blur. These issues necessitate the need to quantify the reticle distortion, induced by the absorption of illumination power, for candidate substrate and coating materials. To study the impact of various substrate and coating materials on reticle performance, detailed three-dimensional transient thermal and solid mechanical models have been developed and extensively applied to predict total placement errors, residual placement errors, and blur on an extreme ultraviolet lithography (EUVL) reticle during scanning. The thermal model includes a bidirectional scanning heat source representative of the illumination incident on the reticle. The heat loads on the reticle are characteristic of an EUVL engineering test stand with a wafer throughput of twenty 200 mm wafers per hour (assuming 80% die coverage and 68% exposure time). This article includes the results which describe the impact of (1) different substrate materials, (2) various degrees of contact conductance between the reticle and chuck, (3) pattern density and arrangement, and (4) temperature variations across the chuck.

The final alignment and characterization of an extreme ultraviolet (EUV) lithographic projection optics system must be performed at the operating wavelength in order to account for errors associated with the combined effect of multilayer coating and substrate fabrication error. Two complementary metrology techniques have been developed which can operate on a compact laser plasma source: EUV lateral shear interferometry (LSI) and aerial image monitoring (AIM). LSI quantifies residual low spatial frequency wave front errors associated with the “figure” of each optical element. To characterize the effect of mid-spatial frequency errors that lead to scatter in the image plane, an EUV aerial image monitor was employed. EUV interferometry was compared to visible light interferometry measurements of the optical system and was found to be in reasonable agreement for low spatial frequency errors. The contrast as measured by AIM and photoresist exposures indicates an additional reduction of contrast associated with...

We have developed a method for encoding phase and amplitude in microscopic computer-generated holograms (microtags) for security applications. An 8 x 8 cell phase-only and an 8 x 8 cell phase-and-amplitude microtag design has been exposed in photoresist by the extreme-ultraviolet (13.4-nm) lithography tool developed at Sandia National Laboratories. Each microtag measures 80 microm x 160 microm and contains features that are 0.2 microm wide. Fraunhofer zone diffraction patterns can be obtained from fabricated microtags without any intervening optics and compare favorably with predicted diffraction patterns.

When characterizing an extreme ultraviolet (EUV) lithographic optical system, visible light interferometry is limited to measuring wave front aberration caused by surface figure error while failing to measure wave front errors induced by the multilayer coatings. This necessitates the development of interferometric techniques at an EUV camera’s operational wavelength (at‐wavelength testing), which is typically around 13 nm. While a laser plasma source (LPS) is being developed as a lithography production source, it has generally been considered that only an undulator located at a synchrotron facility can provide the necessary laserlike point source brightness for EUV interferometry. Although an undulator‐based approach has been successfully demonstrated, it would be advantageous to test a camera in its operational configuration with an LPS. We are developing the latter approach by utilizing extended source size schemes to provide usable flux throughput. A slit mounted at the source plane can provide the nec...

We report the application of direct aerial image monitoring to measure the performance of an extreme ultraviolet lithography (EUVL) test bed. There are several issues which might limit the possibility of performing image monitoring experiments on EUVL systems. These issues include: source flux, signal‐to‐noise ratios, and EUV scanning aperture fabrication. We report the results of initial aerial image scans of equal lines and spaces as well as preliminary measurements of scatter produced by the EUV camera.

Multilayer‐coated imaging systems for extreme ultraviolet (EUV) lithography at 13 nm represent a significant challenge for alignment and characterization. The standard practice of utilizing visible light interferometry fundamentally provides an incomplete picture since this technique fails to account for phase effects induced by the multilayer coating. Thus the development of optical techniques at the functional EUV wavelength is required. We present the development of two EUV optical tests based on Foucault and Ronchi techniques. These relatively simple techniques are extremely sensitive due to the factor of 50 reduction in wavelength. Both techniques were utilized to align a Mo–Si multilayer‐coated Schwarzschild camera. By varying the illumination wavelength, phase shift effects due to the interplay of multilayer coating and incident angle were uniquely detected.

Extreme ultraviolet lithography (EUVL) is a candidate for high‐volume production of integrated circuits with 0.1 μm design rules. As a reduction imaging technique with robust mask substrates, EUVL reduces the mask contribution to the critical dimension (CD) error budget. However, the ability to manufacture EUVL mask blanks that are free of printable defects remains an important challenge. Electromagnetic simulations and imaging experiments have suggested that defects in the substrates and reflective coatings, in particular, may be highly printable and difficult to detect. A defect printability study using programmed defects was performed in order to determine the printing behavior of mask defects of different sizes and locations with respect to absorber features. Imaging was performed using a 10× Schwarzschild camera operating at 13.4 nm with a numerical aperture of 0.08, corresponding to a Rayleigh resolution of 0.1 μm. This system has an effective exposure field of 0.4 mm diam. Measurements of the defec...

The performance of a new 10×‐reduction Schwarzschild system for projection imaging at 13.4 nm wavelength is reported. The optical design is optimized to achieve 0.1 μm resolution over a 0.4 mm image field of view, an increase in area of a factor of 100 over previous designs. An offset aperture, located on the convex primary, defines an unobscured 0.08 numerical aperture. The system is illuminated using extreme ultraviolet (EUV) radiation emitted from a laser plasma source and collected by an ellipsoidal condenser. A 45° turning mirror is used to relay the collected EUV radiation onto a near‐normal reflecting mask. Multiple sets of primary and secondary elements were fabricated, matched, and clocked to minimize the effects of small figure errors on imaging performance. Optical metrology indicates that the wave‐front error within the subaperture used is within a factor of 2 of the design value. Images recorded in poly(methyl methacrylate) and ZEP 520 (Nippon Zeon) resists reveal good imaging fidelity over m...