An electrostatic aberration corrector for improved Low-Voltage SEM imaging.

Spherical and chromatic aberration coefficients Cs and C of various immersion lenses forlow voltage SEM and LEEM are calculated. The minimun values of magnetic immersion lens are C5Cl mm. For the combined electrostatic and magnetic lenses, those values are at most C5=1 mm and CO.7 mm, when the specimen is free from the electrostatic field. Whenthe specimen is immersed in the electrostatic field, those values reduce to C50.2 mm and Cc=o.1 mm at 1 kV. Key words: SEM, LEEM, Objective lens, Spherical aberration, Chromatic aberration, Immersion lens, Cathode lens, Computer aided design 1. INTRODUCTION Importance of low accelerating voltage in scanning electron microscopy (SEM) increasesrecently for semiconductor and biological applications. A main difficulty of low voltage SEM(LVSEM) is its low spacial resolution due to the increase of the wave length and the increaseof AVIV0 ratio where AV is the energy spread and V0 the incident energy. The immersionobjective lens as well as the field emission electron gun is very useful in realizing the highresolution LVSEM. Low energy electron microscopy (LEEM) instruments are very useful forsurface studies of metals and semiconductors. The electrostatic immersion objective lensis essentially necessary for LEEM to get a low landing voltage and low aberrations.There are three kind of immersion lenses: the magnetic immersion lens, the retardingelectrostatic immersion lens and the combination of immersion electrostatic and magneticlenses. The latter two lenses are reviewed by Mullerova and Lenc (1992)1. Here we have to

There is increasing interest in performing x-ray microanalysis of uncoated insulators while operating in unconventional SEM operating modes such as “low voltage” scanning electron microscopy (LVSEM), where the accelerating voltage is ≤ 5 kV and the pressure is low (<10-4 Pa), or variable pressure environmental SEM (VP-ESEM), where a selected gas is maintained at pressures in the range of 1 Pa -1000 Pa. LVSEM and VP-ESEM as microscopy techniques have proven to be extremely successful for imaging uncoated insulators through various charge dissipation mechanisms that are not available under conventional SEM operating conditions (accelerating voltage ≥ 10 kV and pressure < 10-3 Pa). in LVSEM, surface charging of insulators can often be controlled by careful choice of the accelerating voltage, sample tilt, and scan rate, while in VP-ESEM the charged species in the relatively dense gas (ions, secondary electrons) form a self-neutralizing plasma to provide an additional route for discharging the specimen.

Aberration correction in a low voltage SEM by a multipole corrector

Scanning electron microscopy (SEM) of polymers has been a challenge during the past two decades as the conditions required for maximum resolution in the SEM are coincident with conditions which result in maximum beam damage of the specimens. The accelerating voltage requirement for best detail using a tungsten thermionic source is typically about 20 kV, a voltage at which polymers exhibit mass loss, shrinkage and other changes while only providing minimal surface detail. Lower voltages, which produce better surface detail do not provide enough brightness to reveal the details required. The last decade has seen the use of lanthanum hexaboride thermionic sources which have higher brightness and smaller interaction volumes, permitting operation at lower voltages with minimal beam damage and better surface detail due to decreased depth penetration with increased signal to noise. Thus low voltage SEM (LVSEM) is a method used for structure-property studies of polymers.

SummaryDetailed investigating into the effects of spherical and chromatic aberrations, diffraction and the probe current allows the more general formulae for the optimized aperture and the minimum probe radius in low‐voltage scanning electron microscopes (LVSEMs) to be derived using both wave optics and geometric optics. The probe sizes for a diversity of electron sources in LVSEMs have been estimated, which may be useful for practical applications. The computed results indicate the possibility of achieving 1·5–2·0‐nm resolution at low voltages.

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.

The resolution of state-of-the-art low-voltage scanning electron microscopes (LV SEM), which is currently limited by the chromatic and spherical aberrations of the objective lens, can be improved by incorporating an aberration correcting device. At present four different concepts are discussed in literature: Zach and Haider demonstrated that a quadrupole/octupole corrector can correct both chromatic and spherical aberration. Rose proposed a Wien filter for chromatic aberration correction, which has relaxed stability requirements. Recently, we reported a simplified version of this corrector and showed that a spherical aberration corrector can be integrated in a Wien filter. Henstra and co-workers suggested a purely electrostatic corrector that can correct both chromatic and spherical aberration. For all these concepts problems may arise when the lens-to-sample (working) distance for an aligned corrector is to be changed. in general, the corrector settings depend on the ratio Cc/f2, where Cc and f denote the coefficient of the chromatic aberration and the focal length of the objective lens, respectively. When the working distance is changed, this ratio is no longer perfectly matched to the corrector settings. The tedious realignments and readjustments, which then seem necessary, can be avoided by using a doublet objective lens as illustrated schematically in Figure 1.

The low voltage scanning electron microscope (SEM) is widely used in many industrial and research applications due to its ability to image surface details and to minimize charging and beam damage effects on sensitive samples. However, fundamental limitations in beam performance have existed, most notably in the chromatic aberration effects, which become larger as the beam voltage is reduced. The introduction of the extreme high resolution (XHR) SEM has demonstrated that sub-nanometer resolution can be achieved at low beam voltages, revealing fine surface detail. This system uses a source monochromator to reduce the effects of chromatic aberrations, resulting in a more tightly focused electron beam. Beam deceleration is available to provide a further improvement in imaging at low voltages and to give additional flexibility in optimizing the image contrast. While the monochromator is a necessary enabler of the improved imaging performance, further system elements, such as scanning, detectors, stage and environmental controls - that go into completing the SEM - are also key to the usability and throughput when it comes to practical day-to-day performance.

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Images of Ga+ -implanted amorphous silicon layers in a 110 n-type silicon substrate have been collected by a range of detectors in a scanning electron microscope and a helium ion microscope. The effects of the implantation dose and imaging parameters (beam energy, dwell time, etc.) on the image contrast were investigated. We demonstrate a similar relationship for both the helium ion microscope Everhart-Thornley and scanning electron microscope Inlens detectors between the contrast of the images and the Ga+ density and imaging parameters. These results also show that dynamic charging effects have a significant impact on the quantification of the helium ion microscope and scanning electron microscope contrast.

The reduction of integrated circuit dimensions to the submicron range has necessitated the use of low voltage scanning electron microscopy (SEM) for linewidth metrology 1,2. SEM offers advantages of higher resolution and increased depth of focus relative to optical linewidth measurement systems. Low voltage SEM offers a nondestructive method to acquire precise linewidth measurements from features too small for optical systems. In SEM metrology, a primary electron beam traverses the sample surface and the interaction results in a variety of electron signals3. Imaging of wafer topography in the SEM is accomplished by collecting secondary electrons which are emitted from depths of less than about 10 nm of the sample surface. The secondary electron signal is transformed into a video signal as an intensity distribution to be displayed on a CRT. To perform a linewidth measurement, a threshold is selected to determine the distance between the edges of the video profile at that threshold. The threshold technique is critically sensitive to various SEM parameters including the primary electron energy, the beam diameter, and the defocus of the beam4. This sensitivity makes it necessary to calibrate the SEM system after most changes in operating conditions5,6,7. Thus accurate linewidth measurements require an understanding of how the profile and physical dimensions of the feature being measured relate to the video signal. Complications arise because features formed in different materials and on varying substrates result in a variety of video signal profiles. This study was undertaken for the following reasons: *Determine the correlation between low voltage SEM measured linewidth and the physical linewidth of photoresist features on different substrates. *Determine the correlation between low voltage SEM linewidth and physical linewidth for different resist profiles. *Determine the correlation of low voltage SEM linewidth with electrical linewidth measurements performed on polysilicon and aluminum features. *Determine the dependence of low voltage SEM linewidth measurements on wafer tilt and SEM defocus.

Design for an aberration corrected scanning electron microscope using miniature electron mirrors

Wavefront guided ablation

One of the most striking problems in electron optics, the correction of resolution limiting aberrations by means of a corrector incorporated into the electron microscope column, has been solved during the last six years by demonstrating the improvement of resolution beyond the theoretical limit of the uncorrected Electron Microscope (EM). At first, in 1995 [1] with the correction of spherical and chromatic aberration of a dedicated Low Voltage Scanning Electron Microscope (LVSEM) and later, in 1997, with the correction of only spherical aberration of a commercially available 200 kV TEM [2]. The correction of spherical aberration of a dedicated Scanning Transmission Electron Microscope (STEM) at 100 keV primary energy has been demonstrated [3] and further improvements can be anticipated within the near future.These achievements could only be obtained due to the emergence of new computer technology and especially CCD-cameras in the case of TEM correctors. These two developments made it possible first to calculate the electron optical components more precisely and hence, to achieve a better understanding of the requirements on the hardware and second, to have a better computer control of the electron microscope and the corrector itself. The combination of these two new technologies made it possible to go towards an automatisation of the alignment. This simplification of the alignment of an even more complex system is achieved by means of a proper combination of image acquisition and dedicated software in order to analyze and measure the aberrations of an electron optical system on one side and on the other to have appropriate tools to compensate these aberrations by computer controlled power supplies [4,5].

This study aimed to compare ocular wavefront aberrations for pupil diameters of 4 mm and 6 mm, and contrast sensitivity, in eyes with AcrySof IQ and AcrySof Natural intraocular lenses (IOLs). Sixty eyes of 60 patients were enrolled in this prospective randomized study. After phacoemulsification the eyes received either AcrySof IQ SN60WF or AcrySof Natural SN60AT IOLs. One month after surgery, all patients underwent complete ophthalmological examination including corneal topography, wavefront analysis for pupil diameters of 4 mm and 6 mm, and contrast sensitivity measurements with the CSV 1000E instrument under photopic and mesopic conditions with and without glare. There was no statistically significant difference between groups in age, sex or other preoperative ocular characteristics (p > 0.05). Patients with AcrySof IQ IOLs had higher contrast sensitivity at 6 c.p.d. under photopic conditions, at 6 c.p.d. and 18 c.p.d. under mesopic conditions, and at 6 c.p.d., 12 c.p.d. and 18 c.p.d. under mesopic conditions with glare (p < 0.05). Corneal spherical aberration was 0.273 ± 0.074 μm in the AcrySof Natural group and 0.294 ± 0.086 μm in the AcrySof IQ group (p = 0489). Ocular spherical aberration was 0.362 ± 0.141 μm and 0.069 ± 0.043 μm (p < 0.001) for 6-mm diameter pupils and 0.143 ± 0.091 μm and 0.017 ± 0.016 μm (p < 0.001) for 4-mm diameter pupils, with AcrySof Natural and AcrySof IQ IOLs, respectively. There were no significant differences in other higher-order aberrations between the groups (p > 0.05). Aspherical AcrySof IQ IOLs significantly reduced spherical aberration for pupil diameters of both 4 mm and 6 mm and also improved contrast sensitivity more than spherical AcrySof Natural IOLs, especially in mesopic conditions.