Aerial Image Measurement Research Articles

Hazard Assessments of Riverbank Flooding and Backward Flows in Dike-Through Drainage Ditches during Moderate Frequent Flooding Events in the Ningxia Reach of the Upper Yellow River (NRYR)

In this study, the riverbank inundation caused by moderate frequent flooding events (with recurrence periods of less than 20 years), along with the increasingly serious hazards of backward flows in dike-through drainage ditches in the Ningxia Reach of the upper Yellow River (NRYR), were investigated. Then, a comprehensive method for hazard assessment of the floodplains and backward flows in the NRYR was proposed, which fully integrated geographical information systems (GISs), remote sensing (RS), and a digital elevation model (DEM), as well as river dynamics theory. This study first established a one-dimensional unsteady hydrodynamic model for the NRYR. The historical flood hydrology observation from 2012, along with the aerial image measurement data of the study area, were used to calibrate and verify the accuracy of the model. The hazards of riverbank inundation and damages to water affected engineering facilities, as well as the backward flows of dike-through drainage ditches caused by the moderate frequent flooding events, were comprehensively analyzed. Also, this study configured the hazard map and proposed revisions to the flood hazard ranking regime definitions, and discussed the impacts and prevention and control measures of moderate frequent flood damages. The proposed method could effectively meet the hazard analysis demands of the moderate frequent flooding events in the NRYR.

Aerial imaging technology for photomask qualification: from a microscope to a metrology tool

Abstract Photomasks carry the structured information of the chip designs printed with lithography scanners onto wafers. These structures, for the most modern technologies, are enlarged by a factor of 4 with respect to the final circuit design, and 20–60 of these photomasks are needed for the production of a single completed chip used, for example, in computers or cell phones. Lately, designs have been reported to be on the drawing board with close to 100 of these layers. Each of these photomasks will be reproduced onto the wafer several hundred times and typically 5000–50 000 wafers will be produced with each of them. Hence, the photomasks need to be absolutely defect-free to avoid any fatal electrical shortcut in the design or drastic performance degradation. One well-known method in the semiconductor industry is to analyze the aerial image of the photomask in a dedicated tool referred to as Aerial Imaging Measurement System, which emulates the behavior of the respective lithography scanner used for the imaging of the mask. High-end lithography scanners use light with a wavelength of 193 nm and high numerical apertures (NAs) of 1.35 utilizing a water film between the last lens and the resist to be illuminated (immersion scanners). Complex illumination shapes enable the imaging of structures well below the wavelength used. Future lithography scanners will work at a wavelength of 13.5 nm [extreme ultraviolet (EUV)] and require the optical system to work with mirrors in vacuum instead of the classical lenses used in current systems. The exact behavior of these systems is emulated by the Aerial Image Measurement System (AIMS™; a Trademark of Carl Zeiss). With these systems, any position of the photomask can be imaged under the same illumination condition used by the scanners, and hence, a prediction of the printing behavior of any structure can be derived. This system is used by mask manufacturers in their process flow to review critical defects or verify defect repair success. In this paper, we give a short introduction into the lithography roadmap driving the development cycles of the AIMS systems focusing primarily on the complexity of the structures to be reviewed. Second, we describe the basic principle of the AIMS technology and how it is used. The last section is dedicated to the development of the latest generation of the AIMS for EUV, which is cofinanced by several semiconductor companies in order to close a major gap in the mask manufacturing infrastructure and the challenges to be met.

Laser-produced plasma light source for extreme-ultraviolet lithography applications

High-brightness extreme-ultraviolet light sources are required for mask inspections and metrology, including mask blank inspection, actinic pattern inspection, and aerial image measurement system to improve yield and lower cost of ownership. Laser-produced plasma (LPP) light sources have the highest potential to achieve the brightness requirements for all the range of mask inspection tools currently foreseen. High brightness of LPP sources (100 to 1000 W/mm2 sr) is the result of a smaller source size ( ∼ 0.1 mm) than that of competing technologies. Since brightness is inversely proportional to the area of the source, smaller source size corresponds with greater brightness and hence greater inspection throughput. At the Laboratory for Energy Conversion of ETH Zurich, a fully operational continuous-running multi-kHz LPP light source has been developed over the last five years and is now undergoing system optimization. Adlyte, a spin-off of ETH Zurich, is working with industry leaders to commercialize this LPP source. Individual subsystem configuration and the physical boundary conditions and limitations that affect power, brightness, stability, and lifetime management are discussed. This integrated system produces a measured brightness of 259 W/mm2 sr. Outlook for the future growth and integration of the source in high-volume manufacturing tools is then discussed.

Practical application of aerial image by principal component analysis to measure wavefront aberration of lithographic lens

In this paper, we propose a new method that can extract aberrations using aerial image measurements and present its experimental results on lithographic tools. Based on physical simulation and statistical analysis, a linear regression matrix is obtained establishing a connection between principal component coefficients of specific aerial images and Zernike coefficients. In the application phase, the aberrations of the projection lens are solved via the use of this regression matrix. An engineering model is established based on an extension of theoretical model that incorporates all the significant systematic errors. The performance of the engineering model as applied on a 0.75 NA ArF scanner is reported. In the experiment, measurement marks oriented in orthogonal directions are used and aerial images at 9 field points are measured. To verify the repeatability of this technique, every point is measured 20 times. By inputting the aerial images into the engineering model, Zernike coefficients are solved and the results are analyzed. The wafer exposures were performed to evaluate the results of aberration correction.

Modeling of a slit-scan-type aerial image measurement sensor used for optical lithography

Theoretical modeling of a slit-scan-type aerial image measurement sensor used for optical lithography is presented. Slit transmission properties are fully represented by the slit transfer function in terms of incident and scattering angles of light, which is then incorporated into the scheme of a partially coherent imaging formula to obtain an expression for image profiles measured by slit scanning. As an exemplary case, we analyze the influence of a 100 nm width slit used in an ArF lithography system. To understand the mechanism of image profile changes by slit transmission, we focus on frequency transfer characteristics of sinusoidal patterns.

Actinic imaging and evaluation of phase structures on extreme ultraviolet lithography masks

The authors describe the implementation of a phase-retrieval algorithm to reconstruct the phase and complex amplitude of structures on extreme ultraviolet (EUV) lithography masks. Many native defects commonly found on EUV reticles are difficult to detect and review accurately because they have a strong phase component. Understanding the complex amplitude of mask features is essential for predictive modeling of defect printability and defect repair. Besides printing in a stepper, the most accurate way to characterize such defects is with actinic inspection, performed at the design, EUV wavelength. Phase defect and phase structures show a distinct through-focus behavior that enables qualitative evaluation of the object phase from two or more high-resolution intensity measurements. For the first time, the phase of structures and defects on EUV masks were quantitatively reconstructed based on aerial image measurements, using a modified version of a phase-retrieval algorithm developed to test optical phase shifting reticles.

A community-based urban forest inventory using online mapping services and consumer-grade digital images

Community involvement in gathering and submitting spatially referenced data via web mapping applications has recently been gaining momentum. Urban forest inventory data analyzed by programs such as the i-Tree ECO inventory method is a good candidate for such an approach. In this research, we tested the feasibility of using spatially referenced data gathered and submitted by non-professional individuals through a web application to augment urban forest inventory data. We examined the use of close range photogrammetry solutions of images taken by consumer-grade cameras to extract quantitative metric information such as crown diameter, tree heights and trunk diameters. Several tests were performed to evaluate the accuracy of the photogrammetric solutions and to examine their use in addition to existing aerial image data to supplement or partially substitute for standard i-Tree ECO field measurements.Digital images of three sample sites were acquired using different consumer-grade cameras. Several photogrammetric solutions were performed using the acquired image sets. Each model was carried out using a relative orientation process followed by baseline model scaling. Several distances obtained through this solution were compared to the corresponding distances obtained through direct measurements in order to assess the quality of the model scaling approach. Measured i-Tree ECO field plot inventory data, online aerial image measurements and photogrammetric observations were compared. The results demonstrate the potential for using aerial image digitizing in addition to ground images to assist in participatory urban forest inventory efforts.

SLM device for 193 nm lithographic applications

The imaging capability of a new spatial light modulator, (SLM), a custom MEMS device is presented. Low k 1 factor aerial image measurements show the suitability of the SLM device for a variety of uses including optical maskless lithography (OML) applications. Collaborating with ASML partner companies, a fully programmable 11-million micro-mirror SLM device was designed, fabricated, and tested. Innovative MEMS manufacturing techniques were developed to fabricate the very large number of micro-mirrors on the underlying mixed-signal CMOS control circuitry. The individual eight-micron square mirrors were designed to support conventional, attenuated, and alternating phase-shift lithography.

Actinic extreme ultraviolet mask inspection beyond 0.25numericalaperture

The SEMATECH Berkeley actinic inspection tool (AIT) is an extreme ultraviolet (EUV)-wavelength mask inspection microscope designed for direct aerial image measurements and precommercial EUV mask research. Operating on a synchrotron bending magnet beamline, the AIT uses an off-axis Fresnel zoneplate lens to project a high-magnification EUV image directly onto a charge coupled device camera. The authors present the results of recent system upgrades that have improved the imaging resolution, illumination uniformity, and partial coherence. Benchmarking tests show image contrast above 75% for 100nm mask features and significant improvements and across the full range of measured sizes. The zoneplate lens has been replaced by an array of user-selectable zoneplates with higher magnification and numerical aperture (NA) values up to 0.0875, emulating the spatial resolution of a 0.35NA 4× EUV stepper. Illumination uniformity is above 90% for mask areas 2μm wide and smaller. An angle-scanning mirror reduces the high coherence of the synchrotron beamline light source giving measured σ values of approximately 0.125 at 0.0875NA.

Which Mask is Preferred for Sub-60 nm Node Imaging?

ArF immersion lithography may be the best candidate for sub-60 nm device patterning. However, the polarization effect is the most prominent root cause for the degradation of the image quality in high numerical aperture (NA) immersion lithography as the feature size shrinks. Therefore, it is important to understand the polarization effect in the mask. It is common knowledge that a small mask pattern is considered as the wave guide of transmission light. The induced polarization effect shows the different aspects between the conventional mask and the attenuated phase-shift mask (PSM). In this paper, we considered the effects of polarization state as a function of mask properties. The aerial image depends on the polarization states induced by the mask. We evaluated the performances of the conventional mask and the attenuated PSM by using the Solid-ETM simulation and AIMSTM (Aerial Image Measurement System) tool along with real wafer printing.

Optical Properties of LSI Mask Using Cover Substrate

In order to investigate the optical properties of an LSI mask that uses a cover substrate, we have designed a high-numerical aperture (NA), very low aberration reduction projection lens system. On the basis of this design, it was found that the wavefront aberration of the LSI mask that uses a cover substrate between the mask and projection lens is reduced by optimizing of the mask position and imaging position. A phase-shift mask (PSM) using a cover substrate was formed for the experiments. We measured the intensity profiles of this PSM by Zeiss aerial image measurement system (AIMS). It was found that the intensity profiles of this PSM are better than those of the conventional PSM. We have also found that an LSI mask using a cover substrate of specific thickness is able to achieve better image formation using the projection lens system.

High-accuracy aerial image measurement for electron beam projection lithography

A direct means of measuring the image blur of electron beam projection lithography (EPL) tools is described. We developed an aerial image sensor using a Si membrane knife-edge and a transmitted electron detection technique. The aerial image sensor is designed to increase signal amplitude and signal contrast in order to yield a large signal to noise ratio even under a low beam current density condition. The image blur can be quantified accurate to a few nanometers because the measurement error due to the sensor is extremely small. The aerial image sensor was installed in Nikon's electron beam projection experimental column and was evaluated. The measured image blur, defined as the distance between the 12% and 88% points of the beam edge profile, under the optimum condition was 13 nm, and the measurement repeatability was 3 nm (3 sigma). The application of this technique to a system calibration is demonstrated. Focus and astigmatism were measured and the optimum settings of focus coils and stigmators were determined with excellent repeatability. The potential for this technique to provide an automated self-calibration system on EPL tools is clearly shown.

Effective exposure dose measurement technologies: multitone flood exposure by unresolved gratings with multilevel opening ratio

Accurate measurement techniques for effective exposure dose in optical microlithography have been developed. The effective exposure dose can be obtained by a dose monitor mark in a photomask, called the effective dose-meter, consisting of plural segments including grating patterns with a pitch below the resolution limit and different duty ratios. Since the effective dose-meter does not resolve on a wafer but makes flood exposure with the dose as a function of the duty ratio, residual thickness of the photoresist after development changes according to the duty ratio. Therefore, the effective exposure dose can be obtained by grasping the duty ratio of the grating patterns in the effective dose- meter corresponding to the position that the photoresist has completely cleared. The effective exposure dose also can be obtained by measuring the residual thickness of the photoresist at a predetermined segment. A calibration technique utilizing an aerial image measurement system also has been proposed to avoid the influence of intrawafer process variation. The effective dose-meter is small enough for the influence of the intrawafer process variation on the accuracy to be ignored. High dose resolution of less than 0.5% can be achieved completely focus free. The variation of the effective exposure dose in a wafer in the current lithography process was measured to demonstrate this technology.

Performance of multicusp plasma ion source for focused ion beam applications

The multicusp plasma ion source has found many uses such as in ion implanters, ion projection lithography, and injectors for particle accelerators. This source is favorable for such applications because of its low ion temperature, small energy spread, and good stability. These properties also offer the tantalizing possibility of utilizing such a source to create a noncontaminating focused ion beam (FIB) system for semiconductor fabrication line use and advanced photomask repair applications. We have made measurements of the brightness of this ion source and have done some preliminary applications work with a Kr+-enabled FIB system for mask repair and ion milling. A specially designed multicusp plasma ion source has been used to measure focused beam spot size versus current relationships for several inert gas ion species. From these data the axial brightness of the ion source was ascertained. For the extraction of Kr+ from the source we find an angular intensity of 12 μA/sr and a virtual source size of 16.9 μm, giving an approximate value for the image-side brightness (at 30 keV) of 1650 A/cm2 sr. We find good sputtering rates for the Kr+ beam, and the Aerial image measurement system measurements show essentially 100% transmission of 193 nm light through photomask quartz that had been implanted with a significant dose of krypton.

Development and application of a new tool for lithographic mask evaluation, the stepper equivalent Aerial Image Measurement System, AIMS

This paper describes the development of a new tool for evaluating lithographic masks, its software, and its application to the development of advanced mask designs, including phase-shifted features. This mask-imaging system, known as the stepper equivalent Aerial Image Measurement System (AIMS*), provides a means for rapid evaluation of masks. The key feature of AIMS is that the mask is imaged under conditions that emulate the image produced by a given lithographic exposure tool onto a resist layer. In the AIMS microscope, the image obtained is enlarged so as to permit quantitative measurement with a low-noise CCD camera. A quantitative record of selected features of the mask is useful in predicting the printability window for given mask and stepper combinations. Details of the optical system and extensive software capability are given, and examples are presented of feature printability of phase-shifted features, optical proximity, and other effects. Applications include the prediction of key critical mask dimensions as a function of exposure and depth of focus and the rapid checking of the effectiveness of repair actions prior to validation by resist runs. The AIMS microscope system is available as the Carl Zeiss MSM100 Microlithography Simulation Microscope and is now in use in a number of companies as a new tool for mask fabrication and development.

Direct aerial image measurements to evaluate the performance of an extreme ultraviolet projection lithography system

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.

The Application of Deep UV Phase Shifted-Single Layer Halftone Reticles to 256 Mbit Dynamic Random Access Memory Cell Patterns

We have applied deep UV (DUV) halftone reticles, with a single layer absorptive shifter consisting of silicon nitride, to 256 Mbit dynamic random access memory (DRAM) critical levels, and have evaluated the resolution in those cell patterns. For the periodic line levels, halftone reticles were combined with off-axis illumination (OAI) techniques. Resolution capabilities were characterized not only with stepper exposures but also with direct aerial image measurement. For hole levels such the Storage Node and Bitline Contact, halftone reticles offered clear improvement with standard illumination as compared to conventional reticles. For line levels such the Isolation, Wordline and Bitline, dramatic improvement was obtained with the combination of halftone reticles and off-axis illumination.

Printability of sub-150 nm features in x-ray lithography: Theory and experiments

Image formation in x-ray lithography has been studied extensively. A previous theoretical study predicted that 0.1 μm features can be printed at large gaps (≳10 μm) with absorbers attenuating less than 10 dB. This study seeks to verify rigorous electromagnetic simulations of image formation by directly measuring the aerial image. Exposures of features with linewidths ranging from 0.15 to 0.075 μm were performed on the Helios synchrotron. Pedestal-style x-ray masks, consisting of SiNx membranes and a Au absorber, were patterned with e-beam lithography at 100 and 50 kV. By careful dose control and inspection of the resulting features, one can directly determine the aerial image (the image at the resist surface). This is verified using a string model of the resist development. Aerial image measurements correlate reasonably well with modeling results.

Direct aerial image measurement as a method of testing high numerical aperture microlithographic lenses

A method for testing high numerical aperture (NA) microlithographic lenses using direct aerial image measurements has been developed. By measuring contrast versus focus for dense line/space patterns, this system can quantify the amount of astigmatism, distortion, field curvature, and spherical aberration produced by a lens under test. Experimental results show high correlation with more traditional lens testing methods such as transmissive interferometry and evaluating exposed resist patterns. The lens used in these experiments is an 0.53 NA, 248 nm objective designed for imaging 0.35 μm features over a 31 mm diam field.

Etched-quartz fabrication issues for a 0.25 μm phase-shifted dynamic random access memory application

A 0.25 μm pattern from a dynamic random access memory application is used to assess the impact of an etched-quartz fabrication process on the lithographic performance of an actual phase-shifted circuit design. A new measurement tool called the aerial image measurement system is used to quantify the effects of an isotropic wet etch following the quartz reactive-ion etching step on the printability of the phase-shifted pattern at two values of partial coherence. The presence of significant transmission and phase errors, in part a result of optical scattering from the edges and/or corners of the etched-quartz trench, reduced the potential benefits obtained from phase shifting. At the optimum postetch conditions for a partial coherence of 0.36, only 65% of the ideal depth-of-focus was obtained. Transmission errors were reduced from greater than 25% with no postetch treatment to less than 5% with wet-etch depths in excess of 1100 Å. However, large phase errors, between 20° and 60°, were present for all postetch conditions, and were strongly dependent on the partial coherence. Optimization of depth-of-focus required a nominal wet-etch depth of 1200 Å for σ=0.6, but only a 900 Å depth at σ=0.36. Large variations in the phase error as a function of the postetch treatment conditions were attributed to electromagnetic resonance phenomena between the sidewalls of the etched-quartz trench.