A new method for determining the efficiency of laser radiation conversion into 13.5 ± 0.3 nm radiation in laser-plasma extreme ultraviolet sources

We have studied extreme ultraviolet (EUV) emission from a laser-plasma source employing a supersonic xenon jet as the target. The output EUV energy has been determined as a function of the laser pulse energy for the supersonic xenon jet in comparison to the solid metal (Cu, Mo, Ta) targets. Based on these results, a new method for determining the efficiency of laser radiation conversion into EUV radiation is proposed, which ensures unambiguous characterization of the properties of various targets. Ways to optimize the xenon jet source are discussed.     

Xenon capillary discharge extreme-ultraviolet source emitting over a large angular range

We describe a capillary discharge source configuration, allowing for collection of extreme-ultraviolet (EUV) radiation at large off-axis angles, without the need for an EUV window. Operating with xenon gas, the source emits intensely within the EUV spectral region at 11.3 and 13.5 nm. When coupled with a high-collection-efficiency optical system, this source may be suitable for a number of high-average-power EUV imaging applications.     

Determining conditions for 13.5-nm radiation generation by a supersonic-xenon-jet laser plasma source

We propose and justify a simple calculation method for (i) evaluating the optimum conditions for the generation of extreme ultraviolet (EUV) emission by a laser plasma source employing a supersonic xenon jet and (ii) finding ways to increase in the efficiency of such sources. The main processes involved in the interaction of laser radiation with a xenon jet target, which account for the EUV emission with a wavelength of 13.5 nm, are taken into account. It is shown that one of the main factors that decreases the output efficiency of such EUV sources is the absorption of generated radiation by the gas target. Qualitative calculations of the optimum conditions for the generation at 13.5 nm have been performed for three lasers with various wavelengths. The most promising results can be expected for a CO₂ laser, for which the radiation conversion efficiency on the order of 3% at a xenon pressure of 10–15 Torr can be achieved.     

Optical modeling of Fresnel zoneplate microscopes

Defect free masks remain one of the most significant challenges facing the commercialization of extreme ultraviolet (EUV) lithography. Progress on this front requires high-performance wavelength-specific metrology of EUV masks, including high-resolution and aerial-image microscopy performed near the 13.5?nm wavelength. Arguably the most cost-effective and rapid path to proliferating this capability is through the development of Fresnel zoneplate-based microscopes. Given the relative obscurity of such systems, however, modeling tools are not necessarily optimized to deal with them and their imaging properties are poorly understood. Here we present a modeling methodology to analyze zoneplate microscopes based on commercially available optical modeling software and use the technique to investigate the imaging performance of an off-axis EUV microscope design. The modeling predicts that superior performance can be achieved by tilting the zoneplate, making it perpendicular to the chief ray at the center of the field, while designing the zoneplate to explicitly work in that tilted plane. Although the examples presented here are in the realm of EUV mask inspection, the methods described and analysis results are broadly applicable to zoneplate microscopes in general, including full-field soft-x-ray microscopes routinely used in the synchrotron community.     

Tolerancing of diffraction-limited Kirkpatrick-Baez synchrotron beamline optics for extreme-ultraviolet metrology

The recent interest in extreme-ultraviolet (EUV) lithography has led to the development of an array of at-wavelength metrologies implemented on synchrotron beamlines. These beamlines commonly use Kirkpatrick-Baez (K-B) systems consisting of two perpendicular, elliptically bent mirrors in series. To achieve high-efficiency focusing into a small spot, unprecedented fabrication and assembly tolerance is required of these systems. Here we present a detailed error-budget analysis and develop a set of specifications for diffraction-limited performance for the K-B optic operating on the EUV interferometry beamline at Lawrence Berkeley National Laboratory's Advanced Light Source. The specifications are based on code v modeling tools developed explicitly for these optical systems. Although developed for one particular system, the alignment sensitivities presented here are relevant to K-B system designs in general.     

Extreme-ultraviolet lensless Fourier-transform holography

We demonstrate 100-nm-resolution holographic aerial image monitoring based on lensless Fourier-transform holography at extreme-UV (EUV) wavelengths, using synchrotron-based illumination. This method can be used to monitor the coherent imaging performance of EUV lithographic optical systems. The system has been implemented in the EUV phase-shifting point-diffraction interferometer recently developed at Lawrence Berkeley National Laboratory. Here we introduce the idea of the holographic aerial image-recording technique and present imaging performance characterization results for a 10x Schwarzschild objective, a prototype EUV lithographic optic. The results are compared with simulations, and good agreement is obtained. Various object patterns, including phase-shift-enhanced patterns, have been studied. Finally, the application of the holographic aerial image-recording technique to EUV multilayer mask-blank defect characterization is discussed.     

Digital electro-optical imaging sensors

A key source of digital images is the electro-optical imaging sensor. The basic characteristic that identifies a digital electro-optical sensor is conversion of reflected or emitted light from a scene or object into a digital image signal. Examples include television cameras with digital output, and scanning or staring cameras with digital output. We describe general system design techniques to derive optimum imaging sensor specifications, and we offer examples for two applications. One example application is for terrestrial thermal imaging of nearby objects, such as vehicles. The other example is a sensor to create a color image of small targets in low-light, such as an automobile license plate from a distance of several meters.©1993 John Wiley & Sons Inc     

New laser plasma source for extreme-ultraviolet lithography

As the demands of lithographic fabrication of computer chips push toward ever-decreasing feature sizes, projection extreme-ultraviolet (EUV) lithography becomes an increasingly attractive technology. The radiation source of choice for this approach is a laser plasma with a high repetition rate. We report an investigation of a new candidate laser plasma source for EUV lithography that is based on line emission from ice-water targets. This radiation source has the potential to meet all the strict requirements of EUV conversion, debris elimination, operation, and cost for a demonstration lithographic system.     

Sub-diffraction-limited multilayer coatings for the 0.3 numerical aperture micro-exposure tool for extreme ultraviolet lithography

Multilayer coating results are discussed for the primary and secondary mirrors of the micro-exposure tool (MET): a 0.30 NA lithographic imaging system with a 200 microm x 600 microm field of view at the wafer plane, operating in the extreme ultraviolet (EUV) region at an illumination wavelength around 13.4 nm. Mo/Si multilayers were deposited by DC-magnetron sputtering on large-area, curved MET camera substrates. A velocity modulation technique was implemented to consistently achieve multilayer thickness profiles with added figure errors below 0.1 nm rms demonstrating sub-diffraction-limited performance, as defined by the classical diffraction limit of Rayleigh (0.25 waves peak to valley) or Marechal (0.07 waves rms). This work is an experimental demonstration of sub-diffraction- limited multilayer coatings for high-NA EUV imaging systems, which resulted in the highest resolution microfield EUV images to date.     

Materials for terahertz science and technology

Terahertz spectroscopy systems use far-infrared radiation to extract molecular spectral information in an otherwise inaccessible portion of the electromagnetic spectrum. Materials research is an essential component of modern terahertz systems: novel, higher-power terahertz sources rely heavily on new materials such as quantum cascade structures. At the same time, terahertz spectroscopy and imaging provide a powerful tool for the characterization of a broad range of materials, including semiconductors and biomolecules.     

Low-cost, scalable laser scanning module for real-time reflectance and fluorescence confocal microscopy

We present a low-cost, high-speed, retrofitted laser scanning module for microscopy. The cage-mounted system, with various available fiber-coupled sources, offers a real-time imaging alternative to costly commercial systems with capabilities for conventional or confocal reflectance and fluorescence applications as well as advanced laser scanning microscopy implementations. Reflectance images of a resolution target and confocal images of fluorescent polystyrene beads are presented for system characterization. Confocal fluorescence image stacks of T84 epithelial cancer cells are presented to demonstrate application to biological studies. This laser scanning module is a flexible, scalable, high-speed alternative to commercial laser scanning systems suitable for applications requiring a simple imaging tool and for teaching laboratories.     

Applications of the phase transfer function of digital incoherent imaging systems

The phase of the optical transfer function is advocated as an important tool in the characterization of modern incoherent imaging systems. It is shown that knowledge of the phase transfer function (PTF) can benefit a diverse array of applications involving both traditional and computational imaging systems. Areas of potential benefits are discussed, and three applications are presented, demonstrating the utility of the phase of the complex frequency response in practical scenarios. In traditional imaging systems, the PTF is shown via simulation results to be strongly coupled with odd-order aberrations and hence useful in misalignment detection and correction. In computational imaging systems, experimental results confirm that the PTF can be successfully applied to subpixel shift estimation and wavefront coding characterization tasks.     

Laser-produced lithium plasma as a narrow-band extended ultraviolet radiation source for photoelectron spectroscopy

Extended ultraviolet (EUV) emission characteristics of a laser-produced lithium plasma are determined with regard to the requirements of x-ray photoelectron spectroscopy. The main features of interest are spectral distribution, photon flux, bandwidth, source size, and emission duration. Laser-produced lithium plasmas are characterized as emitters of intense narrow-band EUV radiation. It can be estimated that the lithium Lyman-alpha line emission in combination with an ellipsoidal silicon/molybdenum multilayer mirror is a suitable EUV source for an x-ray photoelectron spectroscopy microscope with a 50-meV energy resolution and a 10-mum lateral resolution.     

ECG triggering and gating for ultrasonic small animal imaging

Echocardiography (ECG) is routinely used in the clinical diagnosis of cardiac function. The anatomy of the mouse is similar to that of the human, and thus murine ECG has become an effective tool for the assessment of small animal models of human cardiac diseases. Unfortunately, clinical ultrasonic imaging systems are not suitable for murine cardiac imaging due to their limited spatial and temporal resolutions. Murine ECG requires a spatial resolution better than 100 pim, which mandates the use of high-frequency, ultrasonic imaging (i.e., >20 MHz). High-frequency transducer arrays currently are not available, and so such systems use the mechanical scanning of a single-element transducer for which the frame rate is insufficient for directly monitoring the rapid beating of a mouse heart, and thus retrospective image reconstruction is necessary. This paper presents a high-frequency, ultrasonic imaging system for murine cardiac imaging. Two scanning methods have been developed. One is based on ECG triggering and is called the block scanning mode, in which the murine cardiac images from the isovolumic contraction and isovolumic relaxation phases are retrospectively reconstructed within a relatively short data acquisition time using the ECG R-wave as the trigger to the imaging system. The other method is the line scanning mode based on ECG gating, in which both ECG and ultrasound scan lines are continuously acquired over a longer time, enabling images during the entire cardiac cycle to be obtained. It is demonstrated here that the effective frame rate is determined by the pulse repetition frequency and can be up to 2 kHz in the presented system.     

方波扫描路径确定声源声功率误差特性分析

改进了矩形测量面方波扫描路径测量声功率的误差模型 ,以单极子、偶极子及四极子声源为例 ,给出了扫描方向、测量面离声源的距离及扫描线的密度的选取对理论误差的分布图形。分析结果表明 :对选定的测量面 ,在扫描线密度小的情况下 ,通过调整声源离测量面的距离 ,可以在声源近处找到一个最优点或较优点 ,使理论误差为零或很小 ;在绝大多数情况下 ,沿测量面长边扫描误差比沿短边扫描小。     

Optimized Filtered Back-Projection Tomographic Reconstruction Algorithm for the Step-Shift Scanning of a Sample

The variety of modern X-ray sources opens possibilities to conduct measurements of a wide range of different specimens. The range includes objects with various properties such as the density, the amount and types of defects, and the size which can significantly change from sample to sample. Due to the development of portable high energy X-ray sources like betatrons, it is now possible to investigate objects of big sizes. Although, the fact that the dimensions of such objects can exceed the dimensions of X-ray setups requires to develop new types of scanning geometries and to adopt the algorithms of reconstruction and visualization. The scanning of an object along its longest dimension is one of the ways for investigating such objects. In this article, we present the step-shift scanning approach and the adaptation of filtered back-projection algorithm for this type of measurement. The developed algorithm allows to obtain the visualization of the whole volume of the object from the combination of all scanning steps or to visualize the separate parts of the object from the truncated dataset.     

多排螺旋CT小剂量预扫描门静脉血管成像的临床应用

目的:探讨16排螺旋CT小剂量预扫描门静脉造影的可行性.材料与方法:将100例疑有肝脏疾病进行16排CT门静脉造影的患者分成两组,50例采用常规扫描即注射对比剂后延时25-27s行动脉期扫描,延时45-55s行门脉期扫描,延时80-120s行肝实质期扫描.造影剂浓度370mg/ml,剂量为2ml/kg,注射速率3.5ml/s.50例采用小剂量预扫描即检查均行小剂量预扫描,预扫描造影剂的用量为20ml,注射速率为3.5ml/s.扫描采用轴扫,扫描层厚为5mm,层间距为0mm,时间间隔为2s,扫描层数为8-10层,延时时间为30-35s.获取门静脉时间浓度曲线.确定门静脉扫描的延时时间来进行门脉血管造影.运用容积重建法(VR)、多层面重建法(MPR)和最大密度投影法(MIP)对门静脉及其分支进行重建,评价门静脉成像的效果.成像效果由两位高年资的医生进行评估.结果:采用小剂量预扫描方法的门静脉成像比常规扫描门脉成像成功率高,显示血管细小分支级别高.结论:门静脉小剂量预扫描的方法能大大提高门静脉成像的成功率和较高的门脉成像满意度.具有很好的临床应用价值.     

Observation of Laser-Induced Spark in the Density Jump in a Gas-Jet Target

Technical Physics Letters - In the design of powerful laser-plasma sources of extreme ultraviolet radiation with a gas jet as the target, the problem of degradation of gas nozzles is topical....     

Three-dimensional integral display using plastic optical fibers

A novel approach to an integral imaging system using a pliable plastic optical fiber array is proposed. The proposed system has the advantage that it can utilize a light source for three-dimensional (3D) images at an arbitrary location because the point light sources are formed by the plastic fiber array with flexible optical paths. Two-dimensional images can also be expressed in the proposed system. The light efficiency of this system is high compared with previous point light source array integral imaging systems. The feasibility of the proposed method is explained and demonstrated with experiments.     

Photoacoustic tomography using a Mach-Zehnder interferometer as an acoustic line detector

A three-dimensional photoacoustic imaging method is presented that uses a Mach-Zehnder interferometer for measurement of acoustic waves generated in an object by irradiation with short laser pulses. The signals acquired with the interferometer correspond to line integrals over the acoustic wave field. An algorithm for reconstruction of a three-dimensional image from such signals measured at multiple positions around the object is shown that is a combination of a frequency-domain technique and the inverse Radon transform. From images of a small source scanning across the interferometer beam it is estimated that the spatial resolution of the imaging system is in the range of 100 to about 300 mum, depending on the interferometer beam width and the size of the aperture formed by the scan length divided by the source-detector distance. By taking an image of a phantom it could be shown that the imaging system in its present configuration is capable of producing three-dimensional images of objects with an overall size in the range of several millimeters to centimeters. Strategies are proposed how the technique can be scaled for imaging of smaller objects with higher resolution.