Electron Optical Design Research Articles

An in-lens cryostage for high-resolution secondary-I and scanning transmission electron microscopy

A newly designed cryo-holder was built for the in-lens top stage of a Topcon DS-130 field emission scanning transmission and high resolution secondary electron-I microscope (STEM/HRSEM). The modified Oxford CT-3500 cryo-holder was designed to fit into the narrow bore in-lens of the DS-130F. The cryo-holder was designed to shuttle from the freezing chamber, to the high vacuum chamber of the Denton DV-602 magnetron sputter system for metal coating, and then to the interlock of the DS-130F upper stage. The intention was to provide a single cryo-holder to image frozen specimens by either HRSEM or STEM modes. In this electron optical design, many specimens, prepared by different cryo-preparation procedures, could be imaged by either or both imaging modes. The upper stage of the DS-130F was modified to accept a second liquid nitrogen (LN2) cold trap and a Varian cold cathode ion discharge gauge for monitoring the vacuum at the specimen (Fig. 1).

Diagnostics and electron-optics of a high current electron beam in the Tandem free electron laser — status report

In the construction of the Israeli TANDEM FEL the major task is to develop a high quality electron optic system. The goal is to focus the e-beam to a minimal radius (1 mm) in the interaction region (the wiggler). Furthermore, good focusing throughout the accelerator is essential in order to achieve high transport efficiency avoiding discharge and voltage drop of the high voltage terminal. We have completed the electron optical design and component procurement, including 8 quadrupole lenses 4 steering coils and an electrostatic control system. All are being assembled into the high voltage terminal and controlled by a fiber optic link. Diagnostic means based on fluorescent screens and compact CCD camera cards placed at the HV terminal and at the end of the e-gun injector have been developed. We report first measurements of the beam emittance at the entrance to the Tandem accelerator tube using the {open_quote}pepper pot{close_quote} technique. The experiment consists of passing the 0.5 Amp beam through a thin plate which is perforated with an army of 0.5 mm holes. The spots produced on a fluorescent screen placed 90 cm from the pepper pot were recorded with a CCD camera and a frame grabber. The measured normalizedmore » emittance is lower than 10{pi} mm mR which is quite close to the technical limit of dispenser cathode e-guns of the kind we have. Recent results of the measured transport efficiency and the diagnostics of the high current (1A, 1.5MV) electron-optical system will be reported.« less

Electron optical design of an EFTEM concerning optimum selection of imaging parameters for different sizes of detection systems

The amount of information which can be gained about an object is considerably improved by the incorporation of an imaging energy filter in a TEM. Besides the conventional modes of operation elastic brightfield (EBF) and darkfield (EDF) imaging or diffraction are provided by an Energy Filtering Transmission Electron Microscope (EFTEM) as well as Electron Spectroscopic Imaging (ESI) or Electron Energy Loss Spectroscopy (EELS). Thus, the demands for carefully designed highly flexible electron optics are much more sophisticated as compared to a CTEM.One of the most important parameters for elastic imaging and ESI is the width of the energy window, which can be selected by a slit aperture of adjustable width according to the particular needs. This energy window defines the size of the transferable specimen area or diffraction pattern, respectively, as well as the acceptance angle of the spectrometer. In order to meet the optimum parameters concerning the energy window and the size of the detection system (e.g. photographic plate, image plate, Slow Scan CCD camera (SSCCD), TV camera, electron detector, etc.) the magnifications of the pre- and of the post-spectrometer projector lens systems has to be independently adjustable.

Electron optical design of a high-resolution low-voltage scanning electron microscope with field emission gun

In the present study, a comprehensive description and a complete process for designing a high-resolution low-voltage scanning electron microscope with a field emission gun have been discussed, including the design of two types of magnetic immersion lenses optimized for low-voltage operation, the evaluation of their resolution, the design of the magnetic lens/secondary electron collector system, the calculation of three-dimensional trajectories for both secondary electrons and backscattered electrons, and the estimate of their collection efficiency. The computed results and the primary experiments indicate the possibility of achieving nanometer resolution, which basically approaches the electron optical predictable probe size for the low-voltage scanning electron microscope.

New computer-controlled dual stage SEM—model DS-700

The dual stage electron-optical column design of the DS-700 provides a unique combination of high resolution and flexible sample handling. The top stage accomodates a sample between the polepieces of a low aberration immersion objective lens, while the bottom stage accepts large samples below a conical second objective lens. This column configuration has been described elsewhere, and is therefore not the subject of this paper.A completely new digital electronic system with computer control of all operating parameters has been developed. A combined keyboard and menue control system has been adopted. Commonly used functions are assigned to a simple operating panel, with discrete functions which may be set and easily changed. Secondary functions are addressed using a menue display. In this manner, rapid simple operation is realized, while still providing extreme flexibility.A single high quality TV monitor displays real time and processed images. It is also used to display menues, various operating parameters, and vacuum system status.

Electron-optical design of a high-resolution objective lens for low-voltage Scanning Electron Microscope of biological applications

By providing higher image contrast and reduced charging artifacts, the low voltage scanning electron microscope (LVSEM) is a valuable tool for surface characterization, of particular importance on nonconductive material such as biological specimens. Several SEM designs optimized for use at low voltage have been proposed.Recently, we have designed a new high resolution LVSEM using a field emission gun. The key problem is to decrease both the spherical and chromatic aberration coefficients by using a magnetic lens of small bore diameters(5mm and 10mm) and a narrow gap (7.5mm) (FIG. 1). In our first design, the magnetic lens is built around the side-entry stage of a Philips 300kV TEM and performs as well as that in the present Hitachi SEM H-900 or H-900S. Its simple design has been chosen for reliability and flexibility in farbrication.

GaAsP spin-polarized electron source for a 300 keV accelerator

We report on a GaAsP spin polarized electron source designed to operate in high voltage terminals of electrostatic accelerators of several hundred kV. The small source of simple mechanical and electron-optical design can easily be connected to an accelerator tube. A stable beam of 35%–40% degree of polarization can be produced with a lifetime of several weeks. A measurement of the asymmetry in the emission of bremsstrahlung by transversely polarized electrons has been performed.

Review and outline of environmental SEM at present

SUMMARYThe environmental scanning electron microscope (ESEM) allows the examination of specimens in a gaseous environment. It is based on an integration of efficient differential pumping with a new design of electron optics and detection systems. Backscattered, cathodoluminescence and X‐ray detectors can be designed to fit and to perform optimally in the ESEM. The secondary electron signal can be detected with the gaseous detector device, which is a new multipurpose detector. Insulating, uncoated, wet and generally both treated or untreated specimens can be studied.

Picosecond electron pulse generation via beam deflection-chopping in the SEM

The generation of picosecond electron pulses in the scanning electron microscope (SEM) via beam deflection and chopping with an aperture is analysed with particular reference to energy spread induced in the beam by deflection; three basic deflection-chopping configurations are considered. It is shown that the minimum pulse width attainable at the specimen plane is determined by the beam chopping action and pulse width broadening as a result of inherent and deflection-induced energy spread in the beam, the latter being governed by the beam and deflection parameters. The analysis aims to provide general guidelines for the electron-optical design of picosecond electron beam testing instruments. In a worked example, the trade-offs between beam diameter, pulse duration and average beam current are demonstrated in a performance chart.

A high-duty-cycle long-pulse electron gun for electron accelerators

We describe the design and operation of a long-pulse (200–300 μs), high-duty-cycle (5%–6%), 8-mm-diam dispenser cathode, electrically isolated, modulating Wehnelt electron gun for applications in a high-average-power electron linear accelerator. The electron optics design was optimized with computer modeling of the electron trajectories and equipotentials. The gun performance was established in a series of experimental measurements in a test stand. Excellent pulse-to-pulse emission current reproducibility and electron-beam pulse profile stability were obtained.

Numerical modeling of electron and ion optics on personal computers

Dramatic advances in the power and cheapness of personal computers (PCs) over the past decade, combined with the development of new computer-aided design software, now enable most aspects of electron and ion optical system design to be carried out on PCs. The latest desktop computers, such as the new 80486-based systems and the IBM RISC System/6000, offer computing speeds faster than many mainframes, and enable complete electron optical systems to be designed quickly and cheaply before they are built. The paper describes and illustrates major areas of electron optical design which can now be routinely computed on PCs, including design of individual electron lenses and deflectors, optimization of complete electron or ion optical columns for lithography and inspection (including dynamic focusing and stigmation), discrete Coulomb interactions, analysis of electron guns, multipole systems for beam transport and aberration correction, tolerancing of individual components and complete electron optical columns, design of photomultiplier tubes, fully three-dimensional (3D) field computation and ray tracing in systems with combined electrostatic and magnetic fields, and the 3D modeling of electron-specimen interactions and charging problems in the scanning electron microscope.

Advances in the electron optical design of a multi e-beam comb probe printer

Abstract In order to overcome throughput limitation of sub-μm electron beam lithography a novel multi e-beam comb probe printer which allows direct wafer exposure with patterns with critical dimensions down to 100nm and an exposure speed of 0.1cm 2 /s is under development. This paper describes the present state of the experimental work in fulfilling the high demands of a resolution below 25nm, an extremely low distortion and a uniform current density of 50A/cm 2 in the 100nm probes over the whole length of 200μm of the comb probe.

Techniques of convergent beam electron diffraction.

The techniques required to record standard convergent beam electron diffraction patterns in an analytical electron microscope are discussed in detail, with emphasis on the design of electron optics in commercial instruments. Practical comments are included on specimen preparation, the influence of crystal defects, tilting to major zone axes, and alignment of the instrument. The influence of parameters under experimental control such as probe size, accelerating voltage, temperature, specimen thickness, and convergence angle is discussed in detail. Some comments are included on the alignment and limitations of large angle patterns formed by a defocused probe.

Electron optical design of an e-beam comb probe printer

The electron optical design principles of a 50 keV comb probe printer with 1024 individually switchable 0.1 μm probes in a linear array are outlined. Theoretically the desired resolution of 25 nm and a current density of 50 A/cm 2 can be achieved by applying a LaB 6 linear source, a two lens condenser for forming a parallel ribbon beam and four imaging lenses operated telecentrically in couples for demagnifying the beam defining diaphragm array 100 times onto the water. A new designed source using a prismatically shaped LaB 6 crystal and an experimental imaging column are in operation showing promising results but also a number of practical difficulties which are expected to be governed by increased effort in optical adjustment elements. Particularly a third order compensation by an octopole is unavoidable to achieve the required degree of parallelism of the illumination beam.

Application of variable shaped electron beam technology to the development of 0.5 micron CMOS technology with extensions to 0.25 micron

Over the past several years, CMOS technology has been continuously driven to achieve enhanced performance and higher density. The resulting reduction in semiconductor dimensions has surpassed the limits attainable by the most advanced optical lithography tools. As a result, the utilization of electron beam lithography direct writing techniques to satisfy VLSI patterning requirements has increased significantly. The ability to write patterns at high resolution, however, is by itself insufficient to accomplish the fabrication of VLSI device structures at sub-micron ground rules. The successful application of sub-micron scaling principles to device fabrication requires the complete integration of tool capability and resist process control. In order to achieve the realization of improved CMOS device performance and circuit density, sub-micron ground rules (line width control and overlay) must be satisfied over the full chip. This paper reports on a high performance, fully scaled 0.5μm CMOS technology in which each circuit level is patterned using a modified IBM EL-3 [1] variable shaped electron beam direct write tool. Significant gains in both density and performance at reduced power supply levels (3.3 volts) are obtained over previously reported 1.0μm (5.0 volt) CMOS technology [2]. The details of the innovative, integrated lithography strategy used for this application and the lithography related factors which influence overlay, pattern quality, and device performance will be presented. The modified EL-3 variable shaped electron beam system which was utilized to develop the 0.5μm CMOS technology reported in this paper has been further enhanced to achieve 0.25μm performance capability for both direct write and mask writing applications [3]. Overlay accuracy has been improved through the implementation of low noise electronics and electron optical column design improvements. The application of a 0.25μm variable shaped beam performance capability to the further scaling down of device geometries to the sub-half micron regime and the impact on the integrated lithography process at these reduced dimensions will be discussed.

High-resolution, low-energy beams by means of mirror optics

Electron and ion beam systems with low beam landing energies are needed for many important applications, such as microcircuit inspection and charge storage devices. In this paper, therefore, we have analyzed focusing and deflection systems with strong electrostatic retarding fields immediately in front of the final target plane, with beam landing energies as low as 50 eV. A simple theory is presented, which proves that such strong retarding fields dramatically reduce all the aberration coefficients, including the deflection aberration coefficients as well as the on-axis ones. The overall spot size (for given beam current) inevitably increases as the beam is retarded, due to the increased aperture angle and increased fractional energy spread at the image plane. Nevertheless, on account of the great reduction in the aberration coefficients, the degradation of spot size at low beam landing energies turns out to be much less severe than might intuitively be expected. Our electron optical design software has been enhanced to compute accurately the aberrations of such retarding field systems. This software has been used to verify the predictions of our theory. A practical retarding field system with 50-eV beam landing energy has been built and tested, and its experimental performance confirms our computer analysis. The design has been significantly further improved with the aid of our programs. The computed results show that, using a thermal-field emission source with 1-eV energy spread, the proposed final design should be capable of producing a 111-nA beam of <0.3-μm diameter throughout a 10×10 mm2 deflection field, at a final landing energy of 50 eV.

Picosecond and femtosecond streak cameras: present and future designs

For many applications of time-domain spectroscopy it is desirable to improve the time resolution of electron-optical streak cameras to less than 200 fs in the u.v. visible and i.r. and to ca . 1 ps at X-ray wavelengths. A suite of interactive computer programs has been developed for electron-optical design, taking into account time dispersion. It is possible to analyse rapidly the details of both spatial and temporal imaging properties of currently available streak tubes and to optimize the designs for particular applications. The computed results are shown to be in excellent agreement with the experimental values for the Photochron II streak tube. The expected performance characteristics of a preliminary design for a new tube, the Photochron III, are given.

The U.S. Atom-Resolving Microscope Project

The largest project in transmission electron microscopy ever to be undertaken in this country has recently been initiated within the Materials and Molecular Research Division of the Lawrence Berkeley Laboratory due to the support of the Division of Materials Research, Office of Basic Energy Sciences, U.S. Department of Energy. This project has been given the name “Atomic Resolution Microscopy” and its goal is to provide the instrumentation and expertise necessary to conduct materials research directly at the atomic level. Taking advantage of the best available technology in electron-optical design, its core instrumentation is a unique machine, the Atomic Resolution Microscope (ARM), which is to be built by commercial manufacturers to the specifications of LBL and used in basic research programs requiring atomic imaging capabilities. The project will also include an in-house instrument development effort to maintain state-of-the-art performance from the ARM. Details of history, organization and performance specifications are outlined below.

The inverse interior Dirichlet problem

An infinite number of rotationally symmetrical functions Ψ (r,z) can be found which approximate a given function f (z) on the optical axis 0z (r and z being cylindrical polar coordinates). The Ψ (r,z) are given in the form of a finite Fourier-Bessel series and it is shown how a solution that can be used in electron optical design can be selected by carefully assigning values to the following parameters: the number of terms in the series, the period of the series, and the amount of smoothing introduced. In this way a compromise is established between the quality of the axial approximation, the magnitude of potential gradients to be contended with in zonal regions, and the sizes of apertures allowed. Two-foil, one-foil, and open lenses can be modeled, and given axial potential functions can be accommodated which are either (i) analytic, (ii) continuous but with piecewise continuous z derivatives, or (iii) in the form of a set of experimentally determined values. The method requires no matrix inversion, the computer programming is of a simple nature, and memory requirements are modest enough to allow implementation of the program on small desk-top computers.

An integral field emission “microSEM” for UHV surface analysis

It has been recognized for some time that the application of high sensitivity surface analytical techniques at high spacial resolution is highly desirable. For surface characterization, concentration distribution and topography are superior to average concentration, and sensitivity is increased by localizing the analysis to regions of higher concentration. In the field of Auger electron spectroscopy, this trend is reflected by the increasing popularity of scanning AES (SAM) instrumentation.Due to the low yield of characteristic secondary particles (e.g. Auger electrons or X-rays), the central technological challenge is the delivery of relatively large beam currents into sufficiently small probes (often <30nm). However, electron optical design must also provide for optimum detector collection efficiency, for in many cases beam induced alterations and specimen damage rather than electron optics may limit the useable beam current in small probes.Two basic instrumental configurations are in use; the off-axis approach that treats beam generation and signal detection systems as isolated entities, and the coaxial approach in which the electron gun is an integral part of the detector (e.g. cylindrical mirror analyzer).