Einzel Lens Research Articles

Tandem accel lens advantageous in producing a small spot focused ion beam

This article presents a novel electrostatic lens which is particularly advantageous in producing a smaller spot focused ion beam (FIB). The lens has a tandem structure made up of conventional einzel lenses operated in the acceleration (accel) mode. The structure allows the lens to work under significantly lower excitation voltages compared with conventional accel lenses, making it possible to converge high energy FIB at short focal lengths to minimize aberrations. The minimum spot size obtainable for a 30 keV FIB with 1 pA current has been calculated for various objective lens applications under a restriction where the excitation voltage should be below the beam voltage (30 kV). Spot sizes are 7.3, 6.6, and 4.8 nm for the conventional accel, decel, and tandem accel lenses discussed in this article, respectively.

Development of wide range energy focused ion beam lithography system

A wide range energy focused ion beam system with post objective lens retarding and accelerating field optics has been developed. This system employs two objective lens elements in tandem. The first objective lens element (OL1) is a conventional einzel lens, the second one (OL2) is a retarding and accelerating mode immersion lens. The post objective lens retarding optics is very effective to obtain a low energy fine focused beam. While the post objective lens acceleration optics is not optically attractive, it has other merits. A very high energy ion beam, more than 100 keV, can be obtained by relatively small apparatus. It is also possible to change the landing energy without adjustments of ion beam alignment. The optimized deflector system has been designed for the above mentioned optics. Stitching accuracies of the system were evaluated, and the measured stitching errors are 0.115 μm (2δ) on the X axis and 0.088 μm (2δ) on the Y axis.

Emission Microscope observation of FEA field emission patterns

An emission microscope, which has two electrostatic lenses, has been made at our laboratory in order to observe magnified emission patterns for a FEA (field emission array). The electrostatic lens system consists of a bipotential lens and an Einzel lens (a three electrode static lens). The lenses focus the emission patterns on a phosphor screen placed at 315mm from the center plate of the Einzel lens. The total magnification is 100. The microscope enables to separate the field electron emission from each individual cathode as bright spots on the phosphor screen. It is found that the electron beam from slightly non-concentric structure of the individual cathode deviates from the optical axis and gives a comet-like tailed spot. The luminance of the spots fluctuates like pulses.

Electrostatic accel–decel lens with the advantage of reduced chromatic aberration

An electrostatic lens, operated in accel–decel mode, is presented. The lens has advantage of low chromatic aberration, which is about 2/3 the amount produced by a conventional einzel lens operated in decel mode. The lens has two pieces of electrodes built in to accelerate and decelerate the beam. The decelerating electrode works as a main lens, while the accelerating electrode enhances the beam energy, reducing the aberration. Calculations for the accel–decel lens provide Cc of 22 mm and f of 10 mm with working distance of 5 mm. The lens is suitable for producing a fine beam with a smaller spot size and with energy of over a few tens of kiloelectron volts. When the lens is applied to a 30 keV focused ion beam with 1 pA current, the spot size is estimated as 4.8 nm in diameter on a target with the above working distance.

Projection x-ray microscope powered by highly charged ions

Recombination of slow highly charged ions at the surface of a target foil can be used as a source of x rays for a projection x-ray microscope. In a first test of this concept, a low emittance beam of Ar18+ and Ar17+ ions from an electron beam ion trap was focused with einzel lenses to a 20 μm full width at half maximum spot on a beryllium target foil. The 3 keV x rays from radiative deexcitation of the ions were used to obtain a magnified image of an electroformed nickel mesh with 20 μm resolution by projection onto a CCD camera. Prospects for substantial improvements in resolution and intensity are discussed.

Fabrication of multiple microcolumn array combined with field emission array

We made a multiple electron microcolumn array. The electron source was made of either a Si based field emission array or a nickel coated array to keep the distances among sources equal. Laser micromachining was used to obtain a self-aligned microcolumn with five lenses: an extractor, an accelerator, and an Einzel lens. The aberration of each column was greatly improved compared to anodically bonded Si lens microcolumns. The field emission electron beam pattern was obtained from a multiple microcolumn. Its I–V dependence and possible application to the electron beam lithography with high throughput is discussed.

Detectors for microbeam applications at medium ion energy

Nuclear microprobes with excellent lateral resolution can be realized using a liquid metal ion source and electrostatic einzel lens system in a few hundred keV energy range. In case of ion beam analysis it is better to provide a special detector for Rutherford Backscattering (RBS) analysis or elastic recoil detection. Good mass and depth resolutions should be attained while keeping the damage caused by the beam at tolerable level. Two solutions are considered in the present work: (1) toroidal electrostatic analyzer combined with simultaneous Time-of-Flight (TOF) measurement for mass identification; (2) TOF measurement only, using a large area microchannel plate. Both solutions can be combined with 2D position sensitive anode plates for excellent performance. In case of the first solution, it is possible to distinguish the different mass atoms by the scattering angle where their peak in the angular distribution appears. Similarly to the energy distribution at a fixed scattering angle the angular distribution at a fixed energy represents mass and depth distribution. In case of the second solution the huge acceptance angle will make it possible to analyze submicron spot size without serious beam effects and both heavy and light elements can be investigated.

Fabrication of electron-beam microcolumn aligned by scanning tunneling microscope

Performance of a low-energy electron beam was tested in a scanning tunneling microscope (STM) aligned field-emission microcolumn system made by using silicon microfabricated electrostatic lenses. The system consists of a STM aligned field emitter, an extractor, an accelerator, a beam dump, a quadrupole deflector, and an einzel lens. The beam current of 1 nA with a diameter of ∼0.2 μm was observed when a sample was placed less than 2 mm away from the exiting einzel lens.

A dual vacuum chamber Fourier transform mass spectrometer with rapidly interchangeable FAB, MALDI and ESI sources: electrospray results

A versatile, inexpensive electrospray interface has been constructed and installed on an external source Fourier transform mass spectrometer with a quadrupole ion guide. This interface is readily interchangeable with FAB and MALDI sources, with the main time requirement for changing being the pump-down time. A heated capillary, two skimmers and a set of Einzel lenses are used to deliver ions to the quadrupole ion guide. SIMION (Idaho National Engineering Laboratory, Idaho Falls, Idaho) was used to model ion trajectories and determine optimal dimensions. Five stages of pumping are employed, using mechanical, turbomolecular and cryopumps. A solenoid-driven shutter on the final lens blocks neutrals after injection of the ion packet. Kinetic energy distributions of both small and large biomolecular ions were investigated. For large (10 4–10 5 Da), highly charged species, the kinetic energy distributions were quite favorable to trapping, with sharp peaks at less than 10 eV. Thus a 50 ms ion accumulation time is sufficient to obtain spectra of lysozyme with high S/N ratio. Distributions for smaller ions tended to be somewhat wider and peaked at higher energies. The spectra obtained to date illustrate the ability of the instrument to handle biomolecules ranging from bradykinin to bovine serum albumen.

Experimental evaluation of a 20×20 mm footprint microcolumn

A miniaturized 1 kV electron beam column with a 20×20 mm square footprint for application in arrayed lithography was developed. The actual beam forming optics measured from the electron emitter to the last electrode in the beam focusing Einzel lens is only 3.5 mm in length. The electron source is a miniaturized, high brightness (120 μA/sr), low heating power (<1.5 W) Zr/O/W Schottky field emitter that provides a stable beam current with <1%/h current fluctuations. A custom designed, ultralow profile (0.8 mm high) annular microchannel plate (MCP) detector is fitted into the working distance, which can be varied between 1 and 5 mm, between the Einzel lens and the sample. The MCP provides a high gain, up to 3×104, detector for secondary and backscattered electron detection from solid samples. The beam is scanned over the sample using a prelens double octupole deflector for large field size, ≥100 μm, at low distortions and low deflection aberrations. Using a computer controlled digital pattern generator, patterns with less than 100 nm linewidth were exposed in polymethylmethacrylate resist and reactively ion etched into an underlying tungsten layer.

Construction of STM Aligned Electron Field Emission Source

We constructed a scanning tunneling microscope aligned field emission (SAFE) source by using silicon micro-fabricated electrostatic lenses. The system consists of an STM aligned field emitter, 5μm extractor, 100μm accelerator, beam dump, quadrupole deflector and einzel lens. The microlenses were made by using silicon processing techniques. The system can be operated from 200 to 2 kV, resulting in a beam current of tens of nA and with the diameter of ∼ 0.1 μm when a sample was placed less than 2 mm away from the exiting einzel lens. In order to measure the spherical and chromatic aberrations, a detector and cylindrical electron energy analyzer were attached to the micro-column.

Ion-implantation system for near surface studies

An ion-implantation system designed and fabricated indigenously is described. It consists of a versatile ion source, Einzel lens system, EXH velocity filter, dose measuring equipment, and target chamber. The ion-implantation system fabricated has been used to prepare ion-implanted luminescent phosphors by implanting Sb+ ions into CaS.

Lens and deflector design for microcolumns

The designs of the deflector and the final focusing lens for an electron beam microcolumn are constrained by the extremely small physical size (less than 4 mm overall), and the low energy of the beam (1 keV). Electrostatic lenses are more suitable than magnetic lenses because their field strengths are more practical. For simplicity of construction, the deflector is also electrostatic. It must be designed in concert with the focusing lens to achieve the largest possible deflection field at these low beam energies. Strategies have been developed to optimize the performance of symmetric and unsymmetric Einzel lenses together with deflection systems. The lens and deflector dimensions are varied subject to constraints which ensure that the final design can be fabricated, and nonlinear constraints on the voltages and fields ensure that operation is practical. The properties of symmetric and unsymmetric Einzel lenses have been evaluated. Immersion lenses (in which the beam energy is higher in the column than at the target plane) have also been considered, although they may not be practical in a microcolumn. Another key factor for high performance is the alignment tolerances, since fabrication errors may be a more significant fraction of the lens bore than for conventional lenses. The aberrations produced by misalignment have been predicted for the different lens types, and designs can be selected which can tolerate electrode misalignments of approximately 1 μm for the probe sizes and working distances of interest. The field which can be covered with a prelens double-deflection system is analyzed, and strategies for improving the deflection field are discussed.

An electron-beam microcolumn with improved resolution, beam current, and stability

We have built and tested a 1 keV electron-beam microcolumn that focuses 1 nA of beam current into a 10 nm full width half-maximum beam diameter at a working distance of 1 mm. The electron source is a miniaturized Zr/O/W Schottky field emitter with 150 μA/sr angular emission current density operating at about 1800 K at a distance of only 100 μm from a silicon membrane extractor electrode. The actual microcolumn is 3.5 mm long assembled mainly from silicon membrane electrodes. Improved einzel lens design and fabrication allowed the operation of this beam focusing element in the accelerating mode. Spherical and chromatic aberrations were reduced by factors of about 2–3, respectively, as compared to the retarding lens mode. Excellent beam current stability with less than 1% variation over several hours has been observed.

An electrostatic LEBT for a low-emittance injector: transport and focusing of a high-brightness H − beam from a magnetron ion source

In the context of developing an efficient low-energy beam transport (LEBT) system for a low-emittance injector the problem of transport and matching of a 30 mA, 35 kV H − beam from a magnetron-type ion source is studied; the analysis is done incorporating important experimental constraints. A 30 cm-long LEBT, consisting of six electrostatic quadrupole lenses and a short einzel lens, is designed. Its control parameters are optimized to achieve a solution which matches with the acceptance of a small-aperture radio-frequency quadrupole (RFQ) accelerator. The results of beam dynamics calculations are discussed, and the key issues for emittance growth and its control are acknowledged.

Accurate calculation of geometric aberrations in electrostatic lenses

Using accurate numerical methods, electric fields and meridional electron trajectories have been computed for the three-cylinder einzel lens with sufficient precision to determine values for all the third- and fifth-order geometric aberration coefficients. The third-order coefficients agree very well with those obtained by integration of paraxial trajectories and the fifth-order coefficients are given with estimated accuracy ranging from 5% to 15%. Finally, a study of the applicability of the Renau and Heddle approximation [J. Phys. E 19 (1986) 288] is reported.

Preferential Fragmentation of Protonated Gas-Phase Peptide Ions Adjacent to Acidic Amino Acid Residues

Matrix-assisted laser desorptiodionization (MALDI) is an effective method for producing protonated peptide and protein ions in the gas phase.' Together with electrospray ionization, MALDI has become an ionization method of choice for analyzing peptides and proteins by mass spectrometry (MS).2 The analyses are usually carried out by measuring the molecular masses of the intact polypeptides or peptide fragments produced by proteolysis or chemical degradation of proteins in solution. An alternative method for obtaining detailed primary structural information uses fragmentation of the protonated peptides in the gas phase followed by MS analysis of the fragment ions. A powerful variation of this approach, called tandem MS, employs mass spectrometric selection of the peptide ion of interest prior to fragmentation, followed by analysis of the fragment ion^.^^^ Fragmentation of peptide ions produced by MALDI has been studied in time-of-flight (TOF) mass analyzers (fitted with ion mirrors), instrumentation well-suited to the pulsed ionization ~ource.~*~ Mass analyzers that utilize the principle of ion trapping are also well-suited to the pulsed MALDI ion so~rce.~ Precursor ion selection can be achieved with high resolution.8 The prolonged time window of the measurement in the quadrupole ion trap to =- 1 s) compared with that in the ion mirror TOF analyzer (< s) enables the observation of fragmentation processes with low rate constants. In the reflecting TOF analyzer, highly preferential peptide bond cleavage has been observed at the bond carboxy (C)-terminal to aspartic acid residues, with Asp-Pro bonds exhibiting particularly high lability.6 In the present study, we investigate the fragmentation of MALDI-produced protonated peptide ions (containing an Arg residue and at least one acidic amino acid residue) in a quadrupole ion trap mass spectrometer and also observe preferential cleavage of the peptide bond adjacent to aspartic acid residues. In addition, we report the first observation of preferential fragmentation adjacent to glutamic acid residues. The investigation was carried out on a MALDI quadrupole ion trap mass spectrometer constructed at Rockefeller University? Ions are generated external to the ion trap using 355 nm laser irradiation,I0 accelerated to 60 eV, focused with an einzel lens, and injected axially into the trap. The trapped ions are (1) Hillenkamp, F.; Karas, M.; Beavis, R. C., Chait, B. T. Anal. Chem.

Off-axis injection into an ICR ion trap: a means for efficient capture of a continuous beam of externally generated ions

Highly efficient ion injection into an ICR ion trap of ions generated outside the magnetic field is crucial for successful coupling of a high pressure external ion source, such as electrospray ionization, to an FT-ICR mass spectrometer. Any of several ion transport systems, including r.f.-only quadrupole, r.f.-only octupole, electrostatic einzel lenses, electrostatic ion guide, or use of a supersonic jet expansion to ensure that ions remain virtually on-axis during injection, can overcome the magnetic mirror effect and transport ions at high pressure through a magnetic field gradient toward a low pressure ICR ion trap. However, ions thus transported may exhibit a wide distribution in kinetic energy, making it difficult to trap them all. Gated trapping and pulsed-gas methods are useful for trapping ions from a pulsed source (e.g. laser desorption/ionization), but are not efficient for trapping of continuously injected ions (e.g. electrospray). Here, we introduce a novel method for trapping a continuously generated beam of ions, by injecting ions through a port held at a potential lower than that of the axial front trapping electrode and displaced laterally from the symmetry axis of the trap. Ions reflected from the back trap plate undergo magnetron rotation which in general prevents the ions from exiting back through their injection point. Ion/neutral collisions damp axial motion, and further reduce the ion's chance of escape. Supercomputer simulations of ion trajectories in the three-dimensional electrostatic potential, with collisions modeled as a frictional resistive force, predict efficient trapping of continuously injected ions up to mass-to-charge ratio m/z 50 000 over a wide range (>7 eV) of ion kinetic energy.

Novel electrostatic column for ion projection lithography

Ion projection lithography (IPL) is being considered for high volume sub-0.25-μm lithography. A novel ion-optical column has been designed for exposing 20×20 mm2 fields at 3× reduction from stencil mask to wafer substrates. A diverging lens is realized by using the stencil mask as the first electrode of the ion-optical column. The second and third electrode form an accelerating field lens. The aberrations of the first two lenses (diverging lens and field lens) are compensated by an asymmetric Einzel lens projecting an ion image of the stencil mask openings onto the wafer substrate with better than 2 mrad telecentricity. Less than 30 nm intrafield distortion was calculated within 20×20 mm2 exposure fields. The calculation uncertainty is estimated to be about 10 nm. The calculation holds for helium ions with ≊10 keV ion energy at the stencil mask and 150 keV ion energy at the wafer plane. A virtual ion source size of 10 μm has been assumed. The calculated chromatic aberrations are less than 60 nm, assuming 6 eV energy spread of the ions extracted from a duoplasmatron source. Recently a multicusp ion source has been developed for which preliminary results indicate an energy spread of less than 2 eV. Thus, with a multicusp source chromatic aberrations of less than 20 nm are to be expected. The ion energy at the crossover between the field lens and the asymmetric Einzel lens is 200 keV. Therefore, stochastic space charge induced degradations in resolution can be kept sufficiently low. The divergence of the ion image projected to the wafer plane is less than 2 mrad. Thus, the ‘‘usable’’ depth of focus for the novel ion optics is in the order of 10 μm.

Off-axis injection into an ICR ion trap: a means for efficient capture of a continuous beam of externally generated ions

Highly efficient ion injection into an ICR ion trap of ions generated outside the magnetic field is crucial for successful coupling of a high pressure external ion source, such as electrospray ionization, to an FT-ICR mass spectrometer. Any of several ion transport systems, including r.f.-only quadrupole, r.f.-only octupole, electrostatic einzel lenses, electrostatic ion guide, or use of a supersonic jet expansion to ensure that ions remain virtually on-axis during injection, can overcome the magnetic mirror effect and transport ions at high pressure through a magnetic field gradient toward a low pressure ICR ion trap. However, ions thus transported may exhibit a wide distribution in kinetic energy, making it difficult to trap them all. Gated trapping and pulsed-gas methods are useful for trapping ions from a pulsed source (e.g. laser desorption/ionization), but are not efficient for trapping of continuously injected ions (e.g. electrospray). Here, we introduce a novel method for trapping a continuously generated beam of ions, by injecting ions through a port held at a potential lower than that of the axial front trapping electrode and displaced laterally from the symmetry axis of the trap. Ions reflected from the back trap plate undergo magnetron rotation which in general prevents the ions from exiting back through their injection point. Ion/neutral collisions damp axial motion, and further reduce the ion's chance of escape. Supercomputer simulations of ion trajectories in the three-dimensional electrostatic potential, with collisions modeled as a frictional resistive force, predict efficient trapping of continuously injected ions up to mass-to-charge ratio m/z 50 000 over a wide range (>7 eV) of ion kinetic energy.