Proceedings Volume 2522

Electron-Beam Sources and Charged-Particle Optics

Eric Munro, Henry P. Freund
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Proceedings Volume 2522

Electron-Beam Sources and Charged-Particle Optics

Eric Munro, Henry P. Freund
View the digital version of this volume at SPIE Digital Libarary.

Volume Details

Date Published: 25 September 1995
Contents: 12 Sessions, 54 Papers, 0 Presentations
Conference: SPIE's 1995 International Symposium on Optical Science, Engineering, and Instrumentation 1995
Volume Number: 2522

Table of Contents

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Table of Contents

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  • Electron-Beam Lithography
  • Aberration Theory and Analysis
  • Optimization and Simulation Software
  • Sources
  • Electron-Optical Design
  • Multipole Lenses, Energy Analyzers, Filters, and Sector Magnets
  • Coulomb Interactions and Space Charge
  • Electron-Optical Design
  • Aberration Theory and Analysis
  • Electron-Optical Design
  • Multipole Lenses, Energy Analyzers, Filters, and Sector Magnets
  • Focused Ion Beams
  • Coulomb Interactions and Space Charge
  • Focused Ion Beams
  • Experimental Sources
  • User Facilities
  • Theory
  • Electron-Beam Sources
  • Sources
  • Aberration Theory and Analysis
  • Optimization and Simulation Software
  • Multipole Lenses, Energy Analyzers, Filters, and Sector Magnets
  • Coulomb Interactions and Space Charge
Electron-Beam Lithography
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Electron-beam microcolumn technology and applications
T.H. Philip Chang, Michael G.R. Thomson, M. L. Yu, et al.
A fully functional electron beam microcolumn, 3.5 mm in length, demonstrating a probe size of 10 nm and beam current >= 1 nA at 1 keV has been successfully developed. This paper presents its current status and future directions. Potential applications ranging from low cost scanning electron microscopy to arrays of such microcolumns for lithography, metrology, testing etc. will be discussed.
Electron-optical design for the SCALPEL proof-of-concept tool
Warren K. Waskiewicz, Steven D. Berger, Lloyd R. Harriott, et al.
A proof-of-concept experimental lithography tool, based upon the SCALPELTM principle, is currently being fabricated at AT&T Bell Laboratories. Designed from a lithographic system perspective, we feel that this experimental platform is the minimal configuration necessary to evaluate this specific printing technique, consistent with design rules from 0.18 micrometers to 0.09 micrometers . Details of the machine's electron-optics are presented, including the calculated performance of the electromagnetic lens systems. The critical lithography issues which will be examined using this tool are also discussed.
Large-field electron optics: limitations and enhancements
Hans C. Pfeiffer, Werner Stickel
The renewed interest in charged particle projection lithography has highlighted the need for particle optics to cover larger fields. Previous attempts to develop e-beam and ion beam projection systems for lithography have shown that particle optics can image 108 - 109 pixels in parallel at modest beam currents (< 10 uA). Competitive lithography tools for the high volume production of gigabit chips, however, will have to project more than 1011 pixels at beam currents up to 50 uA. This requirements can only be met through a significant advancement in the state-of-the-art. The field size of electron optics is limited by off-axis aberrations, prohibiting the simultaneous projection of such large numbers of pixels. Therefore, to date the most successful electron beam lithography approach has been that of the probe-forming scanning electron beam system. The sequential exposure of pixels allows (partial) dynamic correction of aberrations and image control `on the fly', but it also severely limits the throughput. Combining, however, the dynamic correction capability of probe- forming systems with the parallel projection of the pixels contained in fractions of a mask appears to be a promising approach to high throughput lithography. It has high potential of meeting the challenges of large field electron optics. This paper describes, how the field size of electron optics can be enlarged despite the limitations posed by off-axis aberrations and Coulomb interactions between beam electrons.
Thermal field emission sources and optics for Gaussian electron-beam lithography
Tom Chisholm, Bernard A. Wallman, Johannes C. Romijn
The use of Schottky emitters has enabled the designers of Gaussian vector-scan e-beam lithography equipment to meet the ever more stringent demands of the semiconductor industry. Although these emitters are expensive, their long life means that they are economical in total cost. Measurements of the influence of noise on the drives to each element of the Leica EBPG- 5FE column show that the only critical parts are the beam alignment units. Stable, low-noise drivers allow the beam positional noise to be within acceptable limits. A numerical technique has been developed to compute the total spot-size in the presence of spherical and chromatic aberrations and this allows the efficiency of the automatic spot optimization routines of the EBPG-5FE to be examined. The present emitter in the EBPG-5FE in DIMES has been in operation since August 1993 and stable and reproducible performance is obtained. The machine is in constant use for a wide range of tasks.
Aberration Theory and Analysis
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Aberration analysis of wide-angle deflectors and lenses by direct ray-tracing and comparison with conventional aberration theories
Eric Munro, Xieqing Zhu, John A. Rouse, et al.
This paper describes numerical techniques for computing aberrations in focusing and deflection systems for charged particle beams, and electron sources and mirrors, by direct ray- tracing of electron or ion trajectories. For such computations to be meaningful and reliable, high accuracy in the computed potential distribution and field components is essential. Suitable methods for potential and field computation in 2D and 3D are outlined, including finite difference and second-order finite element methods, Biot-Savart law and charge density method. A method for direct ray-tracing through these fields is then summarized, using a Runge-Kutta formula. To compare the results of direct ray-tracing with conventional aberration theories, techniques for computing third and fifth-order aberrations using axial field functions and aberration integrals are described. The techniques are illustrated by several examples, including: analysis of spherical and chromatic aberration of an electrostatic lens by direct ray-tracing; analysis of a magnetic probe-forming SEM lens with wide-angle magnetic deflection, with either post-lens or pre-lens deflection; analysis of quadrupole lenses by direct ray-tracing and multipole aberration theory; aberration analysis of an electron mirror by direct ray-tracing; and analysis of the aberration of electron and ion sources.
Dynamic correction of aberrations in focusing and deflection systems with shaped beams
Xieqing Zhu, Haoning Liu, Eric Munro
In electron beam focusing and deflection systems, certain aberrations (such as deflection field curvature, astigmatism and distortion) can be `dynamically corrected', by applying appropriate focusing and stigmation fields corrections at each point in the deflection field. Such corrections are particularly important for large-area shaped-beam systems. In this paper, the question of which aberrations can be dynamically corrected is discussed, and the principle of applying dynamic corrections using dynamic focus coils and stigmators is summarized. The computation of the fields in magnetic and electrostatic stigmators is then described, and a unified aberration theory for handling electron lenses, deflectors, dynamic focus coils and stigmators is outlined. This theory has been implemented and incorporated in an optimization program. An example of the computer-aided design and optimization of a shaped beam system with dynamic corrections is presented. The results lead to the interesting conclusion that, as well as correcting field curvature and astigmatism, dynamic correction elements can also be used to significantly reduce the hybrid distortions of shaped beam systems, thereby largely eliminating pattern distortions and stitching errors.
Rigorous theoretical investigation of distortion in ion-projection noncathode systems
Stanislav N. Jatchmenev, Anatoliy I. Shitakov, Igor P. Vykhat
Calculation technique has been worked out and distortion has been studied in ion optical systems with reduction wide-field image intended to Ion Projection Lithography. It is offered the refined definitions for distortion, fine structure of distortion picture, paraxial system, optimal regime of collimator tuning, which allows us to optimize conditions influencing on distortion value. The criterion of optimization is in maximum work field limited by given value of distortion. Three alternative calculation methods are employed in which third-order and fifth-order distortion is obtained. One is a new approach to the system having collimator and projector. Calculation of electric field is carried out by means of the method of integral equations with calculating derivatives of field functions on axis up to the fourth order which is necessary for fifth-order distortion calculation. Cases of distortion related to perturbation of axial symmetry and their correction and resting members are considered. Rigorous solution of differential equations and analysis of obtained results gives us picture of fine distortion structure with simultaneous presents intrinsic barrel-type and extrinsic pincushion-like areas separated by contour of null-value distortion which due to a deformation in result of presents the fourth-order distortion. Perturbation distortion has been increased in intrinsic area in spite of correction.
Optimization and Simulation Software
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Some problems of mathematical simulation in optimization design of electrostatic image tubes
LiWei Zhou, Zhiquan Zhang, Weiqi Jin, et al.
Three problems of mathematical simulation have been discussed in the present paper when optimizing the electron optical system of electrostatic image tubes. The multigrid method (MGM) is suggested for solving the rotational symmetrical electrostatic field, it proves that the computational efficiency of MGM is 2 - 4 times better than the traditional finite difference method when the same accuracy of iteration is reached. The constrained variable metric method, which ensures the design to have fast convergency, high efficiency and lesser number of times of calling for objective function, is recommended for optimization design. The objective function having a least-square-fit form with weight factors is investigated for the problem of nonlinear multi-objective optimization. The result of optimization appears that the suggested mathematical methods given in the paper for optimization design of electrostatic image tubes are practical and effective ones.
Simulation software BEAMISH for the design of charged-particle optical instruments
Yoshihiro Ueno, Masahiro Takebe, Akihiko Iwata, et al.
The versatile software package BEAMISH has been developed for the design and optimization of charged particle optical instruments. It is possible to pre- and detailed design using this software. There are five distinct features in this software, namely: (1) the optical properties of the same one instrument are calculated by plural computational methods; (2) it is easy to get hold of the optical properties of the instrument by using the various graphical representations; (3) the optimization design can be carried out automatically by the computer; (4) using the Monte Carlo simulation and 3D analysis; it is able to simulate the optical instrument as very real as possible; (5) the optical database is used for the arrangement of computed electromagnetic fields. To realize foregoing distinct features this software package includes the following sub-programs; (a) computations or real or model electromagnetic fields; (b) raytracing and aberration calculations for estimating the optical properties of the instruments; (c) optimization program to minimize the merit functions of the instruments; (d) optical database and its handling programs; (e) graphical interfaces of various computational results and optical properties. In this paper the outline of this software is described and an example (the analysis of Pierce 5B type electron gun) is shown.
Improvements to the electrostatic lens optimization method SOEM
Jim Edmond Barth, H. W. G. van der Steen, Jaroslav Chmelik
Lens optimization can be based on the very fast approximate axial potential calculation of the Second Order Electrode Method (SOEM). In this method the free optimization parameters are the physical quantities of an electrostatic lens system. The optimization, to a merit function based on the optical properties of the lens, is within engineering constraints such as the maximum off-axis field between electrodes. Several improvements to the method have been brought into the program SOEM extending the class of electrostatic lenses that can be optimized. The off-axis geometrical aberrations and the chromatic magnification error can now be included in the merit function. The first and last electrodes can intersect the axis, thus allowing non-field-free spaces at the ends of the lens. Electrode parameters (radii/length/gaps/potential) can be required to be equal giving greater control over the lens configuration freedom in the optimization process.
Primary expert system applied in design of electron-optical system
Wei Lei, Linsu Tong
It is well known that the structure of electron optical system is complicated, so many factors, such as magnification, geometrical aberrations, and space charge effect must be taken into account in the design. At present, the main work of electron optical system CAD is solving equations and calculating numerical values. However, the designs perhaps need more inference and expertise than numerical calculations. In this paper, a primary expert system which is applied in design of electron optical system is established. This expert system is combined with the simulation software SEU-3D program to design some practical electron optical systems. Although the knowledge base is small and rules are not abundant, this paper has used this system to obtain some very useful results. The initial success with this system suggests that further work need to be done whether more rules and knowledge will be added to extend the ability of expert system.
Theory of group applied to calculate field of multielectrode systems with symmetrical geometry of electrodes
Igor Felixovich Spivak-Lavrov
If electrodes of ion optical system possess definite geometrical symmetry then the potential p of the system field can be written in the form of the sum of functions p(alpha ). These functions fall into one of the irresolvable representations of the group for symmetry of field- forming surfaces of the system. Each of the functions p(alpha ) possesses a definite symmetry in relation to the transform of the group and is the solution for a respective boundary value problem. Due to symmetry these new boundary value problems for p(alpha ) are usually simpler than the original problem for the potential p. In more detail is dealt the case, when the ion optical system's field-forming surfaces have the group symmetry Cnv. It is group symmetry of the body with n planes of symmetry penetrating through the axis of symmetry in n-order. For n equals 2,3,4,... it furns quadrapole, hexapole, octapole,... systems respectively. Proposed method allows to receive confined analytical formulas for electrostatic potential of different multipole systems with reference to the width of the clearance between the lamellar electrodes. In so doing, Dirihle's boundary value problem for multi-bound space may be reduced to the solution of Neyman's, Dirihle-Neyman's or Dirihle's boundary value problem for the function p(alpha ), but in a single-bound space what is bound are the two symmetry planes and the part of surface in the original electrode system. We have defined the conditions when such a solution is possible.
Sources
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New design for a field emission electron gun immersed in a magnetic lens field
Kenichi Saito, Hirofumi Morita, Nobuo Shimazu, et al.
This paper describes a new design for a field emission electron gun immersed in a magnetic- lens field. Its low aberration yields both a high brightness and a high emittance. A single-pole- like magnetic lens made a water-cooled pipe coil produces a magnetic flux density of 0.28 T at the emitter tip. The spherical aberration of the gun, as calculated by an electron ray tracing program, is only 1/3 that of a conventional gun with a magnetic lens. A prototype of the gun was constructed using a zirconium-oxide/tungsten thermal-field emitter. The measured crossover diameters agree with those calculated by the ray tracing program, confirming that the aberration is very low.
Comparative study of supertips for electron field emitters
Hans W. P. Koops, Mark A. Weber, J. Urban, et al.
A numerical study of tips and supertips prone for fieldemission sources is performed using a 3D numerical electron optics package. Special supertips are fabricated with additive lithography under computer control. Different materials are used to generate amorphous or nanocrystalline tips. Its performance is simulated. Additive lithography using electron beam induced deposition allows to design base radii from 50 to 1000 nm. Tip radii and tip length of similar dimensions can be generated. Supertips on top of a deposited tip can have a radius as small as 5 nm. This is achieved using a high resolution scanning electron microscope with a cold field emission source. Gold-tips are constructed on top of Pt/Ir-wire tips. The positioning accuracy is 20 nm. Tips are routinely produced with aspect ratios of 5 to 10 and give an additional field enhancement factor. The influence of the nanocrystallinity of the deposited material to the field enhancement is investigated. Nanocrystals at the tip enhance the field up to a factor of 4. This effect explains the high emission current obtained in experiments from nanocrystalline tips.
New MV-class generator
Kiyotaka Ishibashi, Ken-ichi Inoue, Chikara Ichihara, et al.
A new type of high voltage generator has been designed with the aim of developing a compact, inexpensive and easily maintained generator for energetic ion beam analyses such as Rutherford backscattering and particle induced X-ray emission. It consists of several rotating disks and fixed plates made of insulator with metal plates. Since the structure is similar to a rotating disk-type generator, such as the Disktron, it should basically have the potential to display indicators such as compact size and reliable generated voltage stability. Its generating mechanism is, however, rather similar to rectifier-type generators, such as the Cockcroft- Walton and the Schenkel types, so that there are no friction components which might produce a lot of dust, thus opening the high pressure tank for cleaning is an infrequent operation. The proposed generator can be produced relatively inexpensively. A single module model has been built and its generating ability was studied. As a result, it was confirmed that the proposed generating mechanism functions successfully.
Negative electron affinity photocathodes as high-performance electron sources. Part 1: achievement of ultrahigh brightness from an NEA photocathode
Aaron W. Baum, William E. Spicer, Roger Fabian W. Pease, et al.
Brightness, energy spread and emission area are key parameters of electron sources for instruments such as electron microscopes and electron beam lithography tools. In developing transmission-mode NEA photocathodes as sources for these applications, these characteristics have been measured in specialized sealed tubes. Average lateral energies were measured at 63 meV for 1.5 micron thick photocathode, and 83 meV for a 0.5 micron thick photocathode, which was known to be emitting `hot' electrons. A current density of 841 A/cm2 was obtained from a 1.7 micron diameter emission area. This high current density can be explained in terms of lateral drift and diffusion of surface trapped electrons. Combined angular and current density data indicate a brightness of approximately 108 A/cm2-sr at 3 kV.
Electron-Optical Design
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CRT electron-optical system
Shoji Shirai
CRT is the most successful electron optical system, commercially. Over a hundred million systems are produced each year, and distributed to the whole world as television sets or personal computers. Therefore, the system has to be extremely cost and power effective, and ergonomics is the important issue at its design. Also, CRT has to be bright enough to be watched in the luminous living or office room. Therefore, electron beam current and anode voltage (CRT screen voltage) are as high as 0.5 to 7 mA and 20 to 33 kV, respectively. These unique restrictions cause unique electron lens design such as in-line rotationally asymmetrical lens or dynamic quadrupole lens and deflection yoke design such as self converging deflection yoke which produces barrel shaped vertical and pin-cushion shaped horizontal magnetic fields. In this paper the recent technical advancement and future trends of the CRT electron optical system will be discussed. The discussion will be restricted only to the picture tube, and other devices such as camera tube, oscilloscope tube will be excluded.
Immersion lenses for low-voltage SEM and LEEM
Katsushige Tsuno, Nobuo Handa, Sunao Matsumoto
Spherical and chromatic aberration coefficients Cs and Cc of various immersion lenses for low voltage SEM and LEEM are calculated. The minimum values of magnetic immersion lens are Cs equals Cc equals 1 mm. For the combined electrostatic and magnetic lenses, those values are at most Cs equals 1 mm and Cc equals 0.7 mm, when the specimen is free from the electrostatic field. When the specimen is immersed in the electrostatic field, those values reduce to Cs equals 0.2 mm and Cc equals 0.1 mm at 1 kV.
Optimized secondary electron collection in in-lens-type objective lens
Katsuhiro Kuroda, Atsuko Takafuji, Ken-ichi Yamamoto, et al.
Secondary Electron (SE) trajectories were numerically simulated for an ultra-high resolution Hitachi S-900 SEM with an in-lens type objective lens in order to optimize the SE detector position. The trajectories of SEs emitted from the sample were computed as follows. The electrostatic field of SE detector was calculated by a 2D rectangular finite element method (FEM). On the other hand, the magnetic field of the objective lens was calculated by a 2D axially symmetrical FEM. SE trajectories were then 3D simulated by Runge-Kutta method. The SE detector angle relative to the electron optical axis was changed from 45 degree(s) to 60 degree(s) and the SE trajectories were calculated for a various SE positions on the sample at 1 kV to 30 kV accelerating voltage. As a result, the position of the SE detector was optimized so that it gave no variation of brightness even in a low magnification of 250.
Electrostatic lenses with corrected chromatic and spherical aberrations
Lubov A. Baranova, Stella Y. Yavor, Eric Munro
An electrostatic lens is proposed consisting of a set of cylindrical electrodes with gaps, where the axially symmetric, quadrupole and octopole components of the field are generated by a proper choice of electrode potentials. The conditions are found of correction of the chromatic and spherical aberrations in one direction.
Multipole Lenses, Energy Analyzers, Filters, and Sector Magnets
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Pulsed magnetic quadrupole lenses
Hermann Wollnik, Markus Winkler, E. I. Esch, et al.
For heavy-ion inertial-confinement fusion pulsed power techniques are attractive. Pulsed ion beams with a beam duration time of some 10 ns with a repetition rate of some Hz seems to be required. The magnetic flux density in an ion-optical lens must be constant only for a time long as compared to the beam duration. Thus pulsed magnetic lenses are applicable to guide and focus pulsed ion beams. This can save energy and creates strong magnetic fields exceeding several Tesla.
Improvement of sensitivity and depth resolution in conventional RBS and ERDA techniques using energy/momentum filters
Kiyotaka Ishibashi, Yukito Furukawa, Kazushi Yokoyama, et al.
Applications of electric and/or magnetic filters to Rutherford backscattering (RBS) and elastic recoil detection analysis (ERDA) were studied with the aim of improving their performance such as detection limit and depth resolution. A novel RBS technique combined with a charged particle energy filter (CPEF) has been proposed for the detection of contaminants on Si wafers. The CPEF is an electric deflector of charged particles. In this technique, it is placed between a sample and a detector and plays the role of filtering undesirable backscattered signals originated in silicon atoms in the sample from contaminant signals. Experimental results have shown that the CPEF can suppress the silicon signals selectively and appreciably. Another study has been done for ERDA technique using a charge particle energy filter (CPMF) for the analysis of hydrogen depth profile on the surface of a sample. The CPMF is a magnetic deflector of charged particle. It works to remove high energy backscattered particles. As a result, the thickness of the foil in front of a detector can be thinner. The results of calculation have shown that the thickness can be reduced by around 40%.
Curvature of the effective field boundary of realistic sector magnets
B. Pfreundtner, Hermann Wollnik
For realistic sector magnets the effective field boundary is not parallel to the pole face boundary since in the corners the longitudinal and the radial fringing fields overlap. Thus the effective field boundary is always curved even if the pole face boundary is straight. This effect is especially large if the magnet air gap is a large fraction of the radius of curvature of the optic axis. It can be reduced, however, by field clamps and is enlarged by Rogowski pole end shapes. To determine the effective field boundary of realistic sector magnets 3D field calculations are necessary. For the present investigation such 3D fields were determined by the surface-charge method with all saturation effects being neglected.
Split shielding plates in electrostatic sector analyzers and Wien filters
Mikhail Igorevitch Yavor
An analytical method is developed for calculation of the influence of the splitted shielding plates in inhomogeneous electrostatic sector analyzers and Wien filters on their electron optical properties. The method allows one to simplify considerably the choice of the mode of operation of the shielding plates needed to achieve a required electrostatic field distribution inside the analyzer.
Coulomb Interactions and Space Charge
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Solution of electron optics problems with space charge in 2D and 3D
John A. Rouse, Xieqing Zhu, Eric Munro
Space charge effects are significant in many electron optical components, for example CRTs, LaB6 guns and magnetron injection guns. In such devices, the volume space charge influences the beam current and focusing properties. In systems with smaller beam currents, such as e-beam lithography columns, discrete Coulomb interactions cause defocusing, radial blurring and increased energy spread. Various techniques are described for simulation of systems with space charge, including software for modelling rotationally symmetric electron guns with magnetic fields, using second-order finite element method with iterative solution of Poisson's equation. The software has a novel method for the allocation of space charge which simulates the effect of transverse thermal velocities and ensures an even distribution of the space charge. We present finite difference software for 3D systems with space charge, wherein a specified current is associated with each ray, space charge density is assigned to each grid node on the ray path, and then Poisson's equation is iteratively solved for the self- consistent solution. Discrete Coulomb interactions in lithography columns have been modelled with a many-body Monte-Carlo simulation, and various new software features will be described, including the treatment of cell projection systems, and space charge interaction effects in multiple-beam lithography systems.
Electron-Optical Design
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Saturated electron lens design using a second-order finite element method
John Hodkinson, Khadija Tahir
The design of electron optical systems involves the calculation of electromagnetic fields to high accuracy. The first-order finite element method has been extensively used in electron optical design. In the design of magnetic lenses, for instance, discrepancies have been found between computed and measured fields. This discrepancy becomes larger as the saturation level is increased. Rapid variations of permeability with distance causes problems when using a first-order finite element method. These problems are overcome by the application of a second-order finite element method. The method also allows the easier modelling of curved electrodes.
Aberration Theory and Analysis
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Retarding field optics with field-free sample
Laurence S. Hordon, B. B. Boyer, Roger Fabian W. Pease
Retarding field immersion lenses are coming into wide-spread use for focusing low energy electron and ion beams. One disadvantage is that any sample non-flatness will distort the focusing field. A more recent variant is a non-immersion retarding field lens, in which the sample is located outside the retarding field. We describe here a simple model and analysis which predicts that the performance of such a lens is competitive with the retarding immersion lens. With this lens design, it should be possible to focus a 1 keV electron beam with an energy spread of 1 eV into a spot of 4 nm diameter using realistic values of maximum voltage and retarding field strength. The effective resolution under these conditions is estimated to be 1.4 nm.
Electron-Optical Design
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SEIII electrons in the scanning electron microscope: their production, detection, and effect on image quality
Maya Balasubramanyam, Eric Munro
Several types of electrons are produced in a Scanning Electron Microscope during the interaction of the primary beam with the sample. SEIII electrons are those produced when backscattered electrons emitted from the specimen collide with the walls of the specimen chamber and with the polepiece of the objective lens. These electrons constitute a large percentage of the total number of secondary electrons produced, so it is important to understand how they effect the quality of the image. SEIII electrons contain very low resolution information, so it is necessary to suppress them during high resolution imaging. However, the information contained in SEIII electrons relates closely to that contained in the backscattered electrons that are not collected, so when imaging at low magnifications it is possible that the SEIII signal may contain useful information. The work presented in this paper investigates the production and collection of SEIII electrons and, by simulating complete SEM images, the effect on the appearance of the final image can be investigated. This is done by first computing the numbers of electrons emitted from the sample, and hence the number of SEIII electrons produced, using a Monte Carlo simulation. The paths of the electrons within the chamber of the microscope can then be calculated using the software 3DELEC. Various detector configurations are analyzed in order to find the ones that produce optimum results for a range of working conditions.
Multipole Lenses, Energy Analyzers, Filters, and Sector Magnets
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Fringe-field-induced 12-pole component of magnetic quadrupole lenses
B. Pfreundtner, Hermann Wollnik
Energetic ions or electron beams can be focused to very small diameters by the use of electrostatic or magnetic quadrupole lenses or by combinations thereof. In all cases the fringing fields cause image aberrations of third and fifth order that are many times larger than the corresponding aberrations caused by the main field. Though the 12-pole component in the fringing field only influences the fifth order image aberrations their magnitude can be relatively large. For this reason we have investigated the 3D fringing-field distribution for differently cutoff ends of the electrodes or pole faces. It can be shown that by this method the effects of the 12-pole component of the fringing field can be reduced drastically or even be adjusted to compensate corresponding effects of the main field region.
Compact wavelength dispersive x-ray spectrometer for light elements in high-energy ion microprobe system
Yukito Furukawa, Kazushi Yokoyama, Ken-ichi Inoue, et al.
A new styled compact wavelength dispersive X-ray spectrometer for light elements analysis has been developed. This spectrometer was applied for soft X-ray detection (energy less than 1 keV) mounted on a compact high-energy ion microprobe system `mikro-i' and combined with particle-induced X-ray emission in this system. Two multilayer monochrometers, W/Si multilayer and Ni/C multilayer, settled on the compact multilayer benders for X-ray diffraction, and a gas proportional counter with a wide (150 mm X 20 mm) and thin (1 micrometers thickness) polymer film window for soft X-ray made it possible to achieve the compact dimensions of the spectrometer. This system was evaluated for K lines emitted from light elements such as boron, carbon, nitrogen and oxygen. The energy resolution less than 21 eV for each element was observed and was enough to discriminate their characteristic X-rays from disturbing ones. Also, the sensitivity was enough to analyze trace elements or microarea.
Focused Ion Beams
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Use of two focusing modes on two-lens focused ion-beam column
Tohru Ishitani, Y. Kawanami
On focused ion beam (FIB) milling there is a demand for FIBs of high current densities in wide diameter range for high-speed milling with good positional accuracy. There is also a demand for extremely fine FIBs to form scanning ion microscope images with high resolution. Coarse guidelines in designing the two-lens FIB column are obtained from a locus of the characteristic point on beam diameter vs. beam current (d - Ip) curve in logarithmic scale. Here, the characteristic point is the intersection of two lines corresponding to d equals the Gaussian beam spot and d equals the chromatic aberration's spot. The coarse design shows that two focusing modes on the two-lens column is answerable to their demands. The representative FIB specifications of two focusing modes are also described.
Coulomb Interactions and Space Charge
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Coulomb interactions in particle-beam systems
Werner Stickel
The interactions due to the Coulomb force between the constituents of charge particle beam systems have long been explored, with emphasis on the effect of the axial component known as `Boersch effect'. Fewer attempts have been made to quantitatively describe the lateral effects, the bulk space charge effect, which can be compensated by lens adjustment, and the stochastic effect due to the statistical interaction between individual particles. It leads to a non- compensatable, current dependent image blur and is at the root of the failure of direct-write e- beam lithography to become a viable technology for semiconductor manufacturing. It is a key limiting factor of throughput: As throughput goes up with the beam current, so do the interactions and consequently the image blur. Therefore the availability of a theory and/or computer program is essential to predict the interaction effects, before any hardware is built. We give a brief revue of the relevant literature, with emphasis on the treatment of the trajectory displacement effect, the exploration of applicability and limitation of analytical theories, and the application of numerical computer simulation to various system configurations to show the trends and dependencies of particle interactions.
Stochastic interactions in particle-projection systems: comparison of theory and Monte Carlo simulations
Masis M. Mkrtchyan, Steven D. Berger, James Alexander Liddle, et al.
Image blurring as a result of stochastic particle-particle interactions has been investigated for a particle projection system. A comparative analysis of the currently available analytical theories is presented. The results from these theories are also compared with Monte Carlo simulation results. Large variations in results and serious disagreements between the different theoretical approaches are found. The origin of the discrepancies arising from the earlier theories are understood and explained on the basis of our recently developed theory with two key concepts: consideration of nearest-neighbor interactions only, and a randomization length, over which the interactions are correlated.
Calculation of aberrations of a space charge lens and its possible applications
Li Wang, Tiantong Tang, Jon Orloff
Electron space-charge lenses and spherical aberration correctors are studied theoretically. Many interesting features of space charge devices are revealed by considering some simple, but physically reasonable models. A `toroidal sub-beam' algorithm has been developed to model the evolution of the current and charge density distributions of charged particle beams taking into account space charge and initial thermal velocity effects. It is shown that a number of simple space charge distributions can compensate a large range positive spherical aberrations. Some practical structures of the space charge lenses are presented.
Focused Ion Beams
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Low-energy focused ion-beam system for direct deposition
Masahiro Ueda, Shinji Nagamachi, Yasuhiro Yamakage, et al.
Low energy focused ion beam direct deposition has been developed as a new method for fabricating patterned metal films directly on substrate. The principle of this technique is to perform ion beam deposition by using a very low energy focused ion beam. A low energy focused ion beam system for direct deposition has been designed and constructed. In this system the desired material is ionized, separated from undesired ion species, focused, deflected, and decelerated to the optimum deposition energy. The main components of the system are a liquid metal ion source, a mass filter, two Einzel lenses, double octapole deflectors, and an electrically floating sample stage. The beam energy can be continuously varied from 0 eV to 20 keV for single charged ions. The beam diameter can be tuned between 0.5 and 8 micrometers and the beam current varies from 40 pA to 10 nA corresponding to the beam diameter for Au+ ion in the energy range from 30 eV to 200 eV. The purity of deposited gold film was measured by Auger electron spectroscopy and concentrations of carbon and oxygen were below detection limits. The resistivity of gold film was 3.7 +/- 0.1 (mu) (Omega) cm. Currently, new applications of this deposition method are being developed.
Low-energy focused ion-beam system combined with molecular-beam epitaxy system for fabrication of 3D buried semiconductor structures
Junichi Yanagisawa, Kuniyoshi Kito, Kentaro Monden, et al.
Focused ion beam (FIB) system combined with molecular beam epitaxy is of potential importance for fabricating 3D buried semiconductor structures. However, reduction of ion irradiation induced damage, as well as small ion beam diameter, is needed. Low energy FIB system can produce focused beams with a landing energy less than 100 eV by retarding from an accelerating energy of 15 - 25 keV. From the optics simulation performed by Munro code, the beam diameter was estimated to be about 100 nm at a landing energy of 100 eV and a beam current of 60 pA, for an ion source with a current density of 20 (mu) A/sr and an energy spread of 10 eV. In this paper, the characteristics of our low energy FIB system, especially for the beam diameter, was discussed and compared with the calculated result. Direct deposition of Au film by low energy FIB was performed as an example using low energy FIB process and investigated the purity of the deposited film.
Limits on imaging resolution of focused ion-beam systems
Jon Orloff
Focused ion beam systems are capable of very high performance in terms of both beam size and the current density focused on a specimen. Sub-10 nanometer beam size is now routinely obtained with current densities of 1 - 10 A cm-2. The limitations on beam size include beam space charge interactions (radial broadening) as well as the usual source size and system optical aberrations. In addition to these effects the imaging resolution is affected by the signal-to-noise ratio of the ion beam, secondary electron generation and system electronics. Therefore, when calculating imaging resolution it is necessary to take into account the fact that the ion beam is a destructive probe and sputters away the target as it generates information about it. Because imaging resolution depends in a fundamental way on the system signal-to- noise there is a limit on the size of the detail that can be resolved: the beam can destroy an object before an adequate signal from it can be detected. Thus sputtering of the target determines a fundamental limit on the imaging resolution (but not beam size) of a focused ion beam system.
Compensation for rapid contrast variations and correction for charging effects in scanning ion microscopy
Sam T. Davies
Focused ion beam systems are now widely used tools at several stages of semiconductor device production and are finding applications in many other areas. Frequently, it is necessary to combine processing by micromachining or microdeposition with the intrinsic scanning ion microscope function of focused ion beam instruments. A problem in so doing is that image quality can change rapidly during processing as a result of changing secondary electron or secondary ion yields. Moreover, when milling insulating materials, charging effects can give rise to both spatial and temporal variations in contrast. This paper describes a method of achieving closed-loop, automated, compensation for image contrast variations which is also applicable to reducing image degradation due to charging effects in scanning ion microscopy.
Experimental Sources
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Subcompact free-electron laser
John Wesley Lewellen IV, John F. Schmerge, Richard H. Pantell, et al.
The far-infrared portion of the electromagnetic spectrum has remained largely unexplored, partially as a consequence of a lack of suitable sources. The Stanford Far-Infrared Free- Electron Laser has recently demonstrated lasing at 86 micrometers . With component costs, space, shielding requirements, and complexity reduced by up to an order of magnitude from conventional designs, this places the far-infrared free-electron laser within the reach of an individual researcher or a research department. Results from the most recent experimental run are presented. The accelerator section is currently undergoing redesign to make it more suitable for free-electron laser use.
Design and operation of the Compact Infrared Free-Electron Laser (CIRFEL)
Ira S. Lehrman, Jayaram Krishnaswamy, Richard A. Hartley, et al.
The Compact Infrared Free Electron Laser (CIRFEL) was built as part of a joint collaboration between Northrop Grumman and Princeton University to develop FEL's for use by researchers in the materials, medical and physical sciences. The CIRFEL was designed to laser in the Mid-IR and Far-IR regimes with picosecond pulses, megawatt level peak powers and an average power of a few watts. The CIRFEL utilizes an RF photocathode gun to produce high-brightness time synchronized electron bunches. The micropulse separation is 7 nsec which allows a number of relaxation phenomena to be observed. In addition, the photocathode illumination laser can be used in combination with the FEL IR light for pump- probe experiments. The CIRFEL is presently being commissioned and working towards lasing. The present status of the machine is presented.
Radiation produced by relativistic electrons moving over a diffraction grating
John E. Walsh, Kenneth Woods, Richard Fernow, et al.
The characteristics of the emission spectrum produced by a single electron traversing a diffraction grating are explored. Emphasis is on the limit where the electron energy is relativistic.
User Facilities
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Picosecond pump-probe using an FEL and synchrotron source
Karl David Straub, John M. J. Madey, Patrick G. O'Shea, et al.
Two color pump-probe experiments using the Duke Storage Ring as a synchrotron light source for visible light and the Mark III FEL as a tunable, high peak power IR source are possible. The storage ring RF booster and the Mark III FEL RF sources are both driven by the same master oscillator with a timing jitter between sources of less than 20 psec. The operation of the Mark III FEL results in a pulse structure of 2 psec pulses separated by 350 psec contained in a macropulse of 2 microsecond(s) ec duration which is repeated at 10 - 30 Hz. The storage ring can fill from 1 - 64 RF bunches, each with a synchrotron light pulse length of 20 - 30 psec. The circulator frequency for a single bunch is 2.79 MHz. Synchronized operation will result in a coincidence of 6 FEL-IR pulses with the visible synchrotron pulses for each 2 microsecond(s) ec macropulse, or multiples of 6 for multiple bunches in the ring. The visible synchrotron source can be used as a probe of vibrational excitation from the FEL in an experiment using vibrationally-assisted fluorescence as an indicator of overlap of the IR and the visible pulses. An optical delay line in the FEL beam will allow adjustment of the arrival time of the IR pulse relative to the visible probe. Exploration of coupling between electronic excitation and lifetimes of vibrational excitation of fluorescent compounds in solution can be carried out with this configuration.
Short-wavelength light sources at Duke storage ring
Vladimir N. Litvinenko, Genevieve A. Barnett, Bentley Burnham, et al.
A 1.1 GeV electron storage ring is now fully operational at the Duke University Free Electron Laser Laboratory. This ring is dedicated to drive a variety of very high brightness short- wavelength sources ranging from UV to gamma-rays. In this paper we present overview of short-wavelength radiation sources including THE OK-4 (XUV FEL, wiggler radiation and inverse Compton (gamma) -rays), X-ray bend-magnets synchrotron radiation, soft X-ray NIST undulator radiation and hard X-ray inverse Compton source. We also describe status of the sources and our short-term and long-term plans.
NSLS source development laboratory
Ilan Ben-Zvi, Eric Blum, Erik D. Johnson, et al.
The National Synchrotron Light Source (NSLS) has initiated an ambitious project to develop fourth generation radiation sources. To achieve this goal, the Source Development Laboratory (SDL) builds on the experiences gained at the NSLS, and at the highly successful BNL Accelerator Test Facility. The SDL accelerator system will consist of a high brightness short pulse linac, a station for coherent synchrotron and transition radiation experiments, a short bunch storage ring, and an ultra-violet free electron laser utilizing the NISUS wiggler. The electrons will be provided by a laser photocathode gun feeding a 210 MeV S-band electron linac, with magnetic bunch compression at 80 MeV. Electron bunches as short as 100 micrometers with 1 nC charge will be used for pump-probe experiments utilizing coherent transition radiation. Beam will also be injected into a compact storage ring which will be a source of millimeter wave coherent synchrotron radiation. The linac will also serve as the driver for an FEL designed to allow the study of various aspects of single pass amplifiers. The first FEL configuration will be as a self-amplified spontaneous emission FEL at 900 nm. Seeded beam and sub-harmonic seeded beam operations will push the output wavelength below 200 nm. Chirped pulse amplification operation will also be possible, and a planned energy upgrade (by powering a fifth linac section) to 310 MeV will extend the wavelength range of the FEL to below 100 nm.
Theory
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Self-consistent undulator radiation via Lienard-Wiechert fields
Luis R. Elias, Musit Tecimer, Isidoro Kimel
Using Lienard-Wiechert fields and the Lorentz Force relation we present self consistent 3D radiation studies of electron beams moving through periodic electromagnetic structures such as those present in synchrotrons and free-electron laser undulators. Besides providing an economical means of calculating 3D vector radiation fields, our approach yields new insights into individual electron motion as it is driven by both, the velocity fields (Coulomb Fields) and the radiation fields generated by other electrons. We present results of electron beam compression resulting from longitudinal radiation forces competing in opposition with repulsive velocity field forces. We discuss results of noiseless 3D Self Amplified Spontaneous Emission in the X-Ray region resulting from the interaction of a filamentary electron beam with a circularly polarized magnetic undulator.
Design and application of coaxial wigglers in free-electron lasers
Robert H. Jackson, Monica Blank, Henry P. Freund, et al.
The Naval Research Laboratory is investigating innovative magnetic wigglers to reduce beam energy requirements for millimeter wave FELs and to enhance the gain and efficiency. Recent work has focused on coaxial designs. The advantages of this are twofold. First, annular configurations are advantageous for propagating high current beams. The annular geometry permits use of the central structure to enhance the wiggler field, hence, allowing shorter wiggler periods. One such wiggler is referred to as the Coaxial Hybrid Iron (CHI) wiggler, and employs a solenoid enclosing periodic arrays of ferromagnetic and nonferromagnetic material arranged as an outer ring and an inner rod. A second wiggler uses both outer and inner bifilar helical current windings. Both wiggler designs result in substantial enhancements in the wiggler field experienced by the electron beam as compared with the fields in the absence of the central structure. A prototype CHI wiggler is discussed along with a 35 GHz amplifier experiment which is under construction. Preliminary performance calculations for a two helix wiggler system are discussed. This will include both orbit theory and a fully 3D nonlinear simulation of the interaction.
Electron-Beam Sources
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Microwave measurements and beam dynamics simulations of the BNL/SLAC/UCLA emittance-compensated 1.6-cell photocathode rf gun
Dennis T. Palmer, Roger H. Miller, Herman Winick, et al.
A dedicated low energy (2 to 10 MeV) experimental beam line is now under construction at Brookhaven National Laboratory/Accelerator Test Facility (BNL/ATF) for photocathode RF gun testing and photoemission experiments. Microwave measurements of the 1.6 cell photocathode RF gun have been conducted along with beam dynamics simulations of the emittance compensated low energy beam. These simulations indicate that the 1.6 cell photocathode RF gun in combination with solenoidal emittance compensation will be capable of producing a high brightness beam with a normalization rms emittance of (epsilon) n,rms approximately equals 1 (pi) mm mrad. The longitudinal accelerating field Ez has been measured as a function of azimuthal angle in the full cell of the cold test model for the 1.6 cell BNL/SLAC/UCLA #3 S-band RF Gun using a needle rotation/frequency perturbation technique. These measurements were conducted before and after symmetrizing the full cell with a vacuum pump out port and an adjustable short. Two different waveguide to full cell coupling schemes were studied. Experimental and theoretical studies of the field balance versus mode separation were conducted. The dipole mode of the full cell using the (theta) - coupling scheme is an order of magnitude less severe before symmetrization than the Z- coupling scheme. The multi-pole contribution to the longitudinal field asymmetry are calculated using standard Fourier series techniques for both coupling schemes. The Panofsky- Wenzel theorem is used in estimating the transverse emittance due to the multipole components of Ez. Detailed beam dynamics simulations were performed for the 1.6 cell photocathode RF gun injector using a solenoidal emittance compensation technique. The design of the experimental line along with a proposed experimental program using the 1.6 cell photocathode RF gun developed by the BNL/SLAC/UCLA RF gun collaboration is presented. This experimental program includes measurements of beam loading caused by dark current, higher order mode field measurements, integrated and slice emittance measurements using a pepper-pot and RF kicker cavity.
Entropy, reversible and irreversible emittance growth
Patrick G. O'Shea
In this paper we examine the connection between emittance growth and entropy growth in linear accelerators. We divide emittance growth into two classes: reversible and irreversible, depending on the corresponding entropy change. We propose the general hypothesis that if (Delta) E > 0 and (Delta) S equals 0, then the emittance growth may be reversible. We also propose that if (Delta) E > 0 and (Delta) S > 0 then the emittance growth is irreversible. We outline how the concept may be applied to particular cases of relevance e.g. emittance growth and recovery in electron photoinjectors, and wakefield induced emittance growth, where correlations are introduced in the transverse phase space.
Sources
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Micromachined electron gun array (MEGA)
Noel C. MacDonald, Wolfgang Hofmann, Liang-Yuh Chen, et al.
We present the Micro-machined Electron Gun Array (MEGA) as a massively-parallel approach to high throughput production electron beam lithography. The fabrication of the various components of the MEGA are described and the electron optical properties of micro- machined electron guns (MEGs) are studied using electron-optical simulation software. Electron-electron interaction between multiple beams is studied through Monte Carlo simulations for a number of configurations.
Aberration Theory and Analysis
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Realizing any specified image rotation in electrostatic and magnetic imaging
Weiqi Jin, LiWei Zhou, GuoQiang Ni
A theory of electrostatic and magnetic imaging with any specified image rotation, developed from our previous theory of electrostatic and magnetic imaging, has been studied. The conditions of the axial electrostatic potential and magnetic induction for realizing electrostatic and magnetic imaging with any required image rotation are given. According to the given theory, not only an erect image, but also a specific rotated image (or an inverted image) can be realized. As an example, an analytical distribution of axial electrostatic potential and magnetic induction for this purpose is given to realize the electrostatic and magnetic imaging with any specified image rotated angle and magnification M equals 1 or M $NEQ 1.
Optimization and Simulation Software
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Simulation of electron-optical system in color monitor tube
Wei Lei, Linsu Tong
In color monitor tube, electron optical system influences the quality of whole tube directly. With the development of HDTV and technology of graphic display in workstation, the quality of color monitor (display) tube must be improved. For improving the resolution, the axi- symmetric electron optical system is adopted to decrease the size of spot on screen. Because the structure of axi-symmetric electron optical system is complicated, it is impossible to analyze it with analytical method. This paper uses SEU-3D program to simulate electron optical system in color monitor tube. Taking account of the characters of electron optical system in color monitor tube, this program uses some special methods to analyze the system so that the result obtained from the simulation can fit the actual condition properly.
Numerical techniques and software for static and dynamic image tubes design
Mikhail A. Monastyrski, Sergei V. Andreev, Valentina P. Degtyareva, et al.
The results of many-years activity in elaboration of numerical techniques and software for static and dynamic image tube design are presented. The developed software is based on the first kind integral equations technique, aberration theory, and special algorithms which allow high-precise image quality computation. The software was carefully tested and used in numerous calculations for spatial and temporal parameters of image tubes operating in static and dynamic modes. Some examples of these calculations are graphically presented and discussed in details.
Multipole Lenses, Energy Analyzers, Filters, and Sector Magnets
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Transfer matrix of a narrow gap between two quadrupole lenses
Mikhail Igorevitch Yavor, O. A. Grineva
The field integral method is developed in the third-order approximation to calculate effects of transformation of charged particle trajectories in the gaps between electrostatic or magnetic quadrapole lenses in multiplets.
Coulomb Interactions and Space Charge
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Global space charge effects in ion-beam lithography
John J. Petillo, Alfred A. Mondelli
Ion Projection Lithography (IPL) is a candidate technology for meeting expected circuit design rules (less than 0.18 micrometers) for future generations of semiconductor devices. The Advanced Lithography Group (ALG), a consortium of industrial and US government laboratories and universities, is developing IPL technology with funding from ARPA and the state of Maryland. A prototype IPL device, the ALG-1000, is being developed to demonstrate the capability of IPL to meet future requirements for pattern overlay. To realize IPL technology requires control of space charge effects in the ion optical column. Due to the length of the IPL system (several meters), the precision of the calculation to predict distortion at the wafer plane becomes difficult to perform. Including the effects of space charge effects is even more difficult. Both global and stochastic space charge phenomena occur. This paper presents a system of computer models that allows simulations of both global and stochastic space charge effects. In particular, the models will be used on the ion beam projector in the ALG-1000 device. The calculations are carried out using a self-consistent equilibrium ray tracing code, where the applied fields are calculated from the actual lens column geometry. For global space-charge, the model also includes optimization of the lens electrode voltages to minimize pattern distortion at the wafer plane.