Corrected Lens Systems Research Articles

荷電ビーム用多段四極子補正レンズの高精度アライメント技術

In charged particle optics, high-precision alignment of multistage multipole lens is essential for the realization of a non-rotationally symmetrical correction-lens system. A six-stage self-aligned quadrupole correction-lens system consisting of six quadrupoles and four aperture electrodes was designed and constructed. Machining accuracy and high-precision alignment techniques for the multistage correction-lens elements are reported in this article. The correction-lens system is assembled using cylindrical components, which can be machined with high accuracy. To overcome the problem of azimuthal error in the multistage quadrupole lens, the correction-lens system consists of two lens-casing cylinders of the same shape and size. To suppress azimuthal error among three quadrupoles in a lens-casing cylinder, four long positioning shafts and eight ceramic insulating shafts are inscribed in this cylinder. Cylindricity of the positioning shafts is less than 0.2μm over a length of 65 mm. Quadrupole electrodes are mounted on four pairs of ceramic insulating shafts using a Z-axis positioning jig. All sets of quadrupole electrodes are finished with varying diameters and lengths less than 0.15μm. The constructed multistage quadrupole correction lens is installed as an objective lens in a focused ion beam system, and azimuthal error between two lens-casing cylinders are corrected by using scanning ion microscope images.

High-Precision Quadrupole Correction Lens for Ion-Beam Focusing: Design and Test of Multistage Self-Aligned Quadrupole Correction-Lens System

A multistage self-aligned quadrupole correction lens, which consists of two lens casings, is developed. Each lens-casing cylinder consists of three quadrupole and two aperture electrodes. All lens components in lens-casing cylinders are fabricated to achieve an electrode alignment accuracy better than 1 µm. Azimuth errors between entrance and exit lens-casing cylinders can be corrected using the rotary flange fastened to the entrance lens-casing cylinder. The correction-lens system is installed as the probe-forming objective lens in place of the rotationally symmetrical objective lens of the focused ion-beam system. Simulation results show that a four-stage quadrupole correction lens is useful as a correctable probe-forming objective lens because all aperture aberration coefficients can be effectively corrected under the condition of relatively low chromatic aberration. Probe-forming properties and beam shape affected by aperture aberrations of the four-stage quadrupole correction-lens system are observed by scanning ion microscope images.

High-Precision Quadrupole Correction Lens for Ion-Beam Focusing: Design and Test of Multistage Self-Aligned Quadrupole Correction-Lens System

High-Precision Quadrupole Correction Lens for Ion-Beam Focusing: Design and Test of Multistage Self-Aligned Quadrupole Correction-Lens System

Third-order spherical aberration correction using multistage self-aligned quadrupole correction-lens systems

New multistage self-aligned quadrupole correction-lens systems are proposed for correcting the spherical aberration of a rotationally symmetrical lens in a probe-forming system such as electron beam lithography and focused ion beam. These multistage correction-lens systems consist of six- or eight-stage electrostatic quadrupole and aperture electrodes placed between the quadrupoles. An octupole field for the correction of aperture aberration is automatically created and aligned with a quadrupole field by supplying a voltage to the aperture electrode. The optical properties of the self-aligned quadrupole correction-lens systems are precisely simulated using the potential functions approximated from the calculated three-dimensional potential distributions. The lens components of the correction-lens systems are symmetric with respect to the mid-plane of the correction system, and the quadrupole excitations are anti-symmetric to the mid-plane. The simulated optical properties of the six- and eight-stage self-aligned quadrupole correction-lens systems are compared with a four-stage self-aligned quadrupole correction-lens system. Aperture aberration coefficients of the six- or eight-stage quadrupole system under non-excitation of the aperture electrodes become much smaller than those of the four-stage quadrupole system. It is found that the correction of spherical aberration using the six- or eight-stage self-aligned quadrupole correction-lens system can be easily achieved under the condition of considerably lower excitation of lens elements in comparison to the four-stage self-aligned quadrupole correction-lens system.

EPL electron optics performance on test stand.: 2. Field distortion and stitching results

Nikon in collaboration with IBM has been developing electron projection lithography (EPL) as a promising candidate for 65 nm node technologies. In this paper, the latest performance of electron optics for a 65 nm R&D tool will be described with emphases on field stitching and distortions. For evaluation, data measured by a Nikon XY5i metrology tool, SEM pictures of visual vernier patterns, and other data will be presented to show field stitching results. These represent the first full main field stitching results achieved. To obtain stitching accuracy between any two adjacent subfields, it is critical to eliminate subfield distortion at each deflected position. Subfield distortion is mainly reduced by a combination of optimized curvilinear variable axis lens (CVAL) deflection optics and dynamic correction lenses adjustment. We will discuss an experimental method to minimize subfield distortion by adjusting the CVAL deflection optics at deflected positions. In addition, dynamic correction of the subfield image shape eliminates linear components of subfield distortion. Controllability of subfield shape will be discussed. As another important topic, we will present results of nonlinear distortion correction by a higher order dynamic correction lens system. By combining this with the above mentioned electron optical adjustment, we achieved less than 10 nm residual subfield distortion at the most deflected position. We will also present the wide ‘tolerance’ or ‘margin’ of nonlinear distortion correction adjustment.

A new type of quadrupole correction lens for electron-beam lithography

A new type of quadrupole correction-lens system with lower aperture aberration coefficients has been developed for an electron-beam lithography system. The new lens is called a self-aligned quadrupole correction lens and consists of three-stage electrostatic quadrupoles, and aperture electrodes placed between quadrupoles. An octupole field for correction of aperture aberration is automatically created and aligned with a quadrupole field by supplying a voltage to the aperture electrode, and it has its peak near the edge portion of the quadrupole. A new mechanical construction of the quadrupole correction-lens system is developed for realizing high-precision alignment of the multistage lens electrodes. The optical properties of the new correction lens are exactly calculated by using simulated potential distributions. Aperture aberration coefficients of the new correction lens can be corrected to less than 0.1 mm. It is confirmed experimentally that the probe size is improved remarkably by excitations of the aperture electrodes.

Radial and Axial Turbulent Flow Measurements With an LDV in an Axisymmetric Sudden Expansion Air Flow

Radial velocity component measurements in cylindrical tubes have been difficult to make because of optical aberrations introduced by the curved tube wall. This is particularly troublesome in gas flows where refractive index matching techniques cannot be employed. The present investigation utilized a specially designed correction lens system to overcome this problem. As a result it was possible to map the axial and radial velocity behavior in detail for the air flow downstream of a sudden expansion in a cylindrical duct. Quantities measured and derived included mean velocities, turbulence intensities, turbulent kinetic energy and Reynolds stress. The weak secondary recirculation zone existing just below the sudden expansion was clearly identified and mapped. Where possible the measurements were compared with numerical predictions based on a k-ε model.

Geometric aberration correction in ion lenses - the Caledonian quadruplet

In a recent publication a novel focussing element for charged particle beams was introduced and preliminary results were presented which indicated that the element was suitable for designing aberration corrected lens systems. In this paper we present further results which show that the aberration caused by parallel rays not coming to a unique focus can be corrected between two such elements in a configuration nicknamed the Caledonian Quadruplet. Another principal aberration results from ray bundles approaching at different angles not coming to a focus on a single plane. We outline our continuing work to simultaneously correct both these aberrations with a hexuplet system.

The effect of multiple diffraction on the electron-microscope image

It is pointed out that multiple scattering in the specimen must affect the accuracy of the images produced in electron microscopes. A simple line of reasoning is offered which shows that, even with highly corrected lens systems, resolutions better than about 30 Å are unlikely to be produced for specimens of 1000 Å thickness. For increased resolution, thinner specimens or reflection microscopes should be used.

Limitations to Resolving Power in the Thermionic Emission Microscope†

ABSTRACT Formulation of resolving power as a function of electron chromatism at the source only cannot account for the large deviation from practical resolution figures measured for emission microscopes, not even when best corrected lens systems and very high accelerating potentials are employed. Considering electron diffraction only as the decisive resolution limiting factor, too low a value is obtained. Hence, it appears, that the optimum practical resolution of 800 Å would lie between the two functions. An approximation is given for computing the effective resolving power. It demonstrates the little influence of the field strength at the emitter surface on overall resolution. This then also implies that the use of very high first anode voltages add little only to enhance image quality. It is shown how severe the resolution degrading property of surface irregularities is as evidenced by photographs taken from a 21 in. directly viewed screen. Surface depressions, elevations and pores may lead to serious misinterpretations of the actually existing emission mechanism due to electron ray refraction by distorted equipotential planes which shape the trajectories in peculiar ways. Bunching, focusing and divergence effects are shown to influence the emission patterns on the screen, also preparing the way for a proper interpretation of observations on more difficult surfaces such as oxide cathodes or any other irregular surface. Special situations are also discussed which produce electron-optical illusions on the screen such as loss of surface detail, splitting of electron sheets, multi-helical paths and multi-circular patterns.

The Direct Calculation of Achromatized and Corrected Optical Systems*

The design of complex lens systems is a tiresome and time-consuming work of successive trigonometric light path calculations, each leading to a new lens bending, air space modification, etc. It can take thousands of man-hours.A direct process which gives at once the proper lens bending (perhaps the best) and the surfaces’ radii, cuts the design work to a small fraction of the classical calculation time. This is the case in this direct process for which a few initial hints are given here.When the primitive data are related by geometric optics to the entrance and exit pupils, field, aperture, object and image positions, this process leads at once to right results. Generally, they are already good for practical use; when extremely corrected lens systems are needed, it happens that only one partial bending is sufficient to compensate the residual aberrations.Basis of this new design process is a new equation for achromatization into which enter the lens thicknesses, air spaces, refraction indexes, image position. When data are properly chosen, it is a very rapid job to get the curvature radii of all the optical surfaces.