Axial resolution of confocal Raman microscopes: Gaussian beam theory and practice

A straightforward and transparent model, based on Gaussian beam optics, for the axial r ₀ resolution of a confocal microscope is presented. A confocal Raman microscope was used to determine the axial confocality in practice. The axial response of a thin planar object was measured for three different objectives, two pinhole sizes and a slit. The results show that, in the case of a confocal configuration, the response calculated with the model provides a good prediction of the axial resolution of the confocal microscope.     

Imaging in scanning microscopes with slit-shaped detectors

We consider the imaging in scanning microscopes employing point and slit-shaped detectors in both the bright-field and fluorescent mode. In particular we consider the three-dimensional aspects of the imaging and show inter alia that acceptable, albeit asymmetrical, images result from a slit detector system at low levels of defocus. The situation becomes worse as the defocus is increased although acceptable extended-focus and isometric images are still possible in some cases.     

Intercomparison of scanning probe microscopes

Comparison measurements on reference standards are reported in which 13 partners with different instruments took part. A set of prototype standards which had been produced and calibrated within a European project were used for the measurements. Here, results of measurements on a 240 nm step height standard and a two-dimensional lateral standard with a nominal pitch of 1 μm are reported.     

Increasing the scanning speed of scanning tunnelling microscopes

The scanning speed and quality of images obtained with a scanning tunnelling microscope can be increased by replacing the tunnelling tip holder with an active, high resonance frequency piezoelectric positioner. The combination tip holder and positioner has a resonance frequency near 100 kHz, allowing fast closed-loop feedback control of the STM. With appropriate electronic shielding, the active tip holder can also be used to modulate the tip to surface distance at high frequencies for work function mapping. The high speed positioner is complemented by a ‘tip-saver’ circuit which senses imminent tip-surface contact, preventing tip crashes. The circuit, when driving the fast positioner, has a response time of under 20 μs and is not subject to the oscillation problems normally associated with closed-loop feedback systems operated at high gains and large signal bandwidths.     

MEMS-based fast scanning probe microscopes

Scanning probe microscopy is a frequently used nanometer-scale surface investigation technique. Unfortunately, its applicability is limited by the relatively low image acquisition speed, typically seconds to minutes per image. Higher imaging speeds are desirable for rapid inspection of samples and for the study of a range of dynamic surface processes, such as catalysis and crystal growth. We have designed a new high-speed scanning probe microscope (SPM) based on micro-electro mechanical systems (MEMS). MEMS are small, typically micrometer size devices that can be designed to perform the scanning motion required in an SPM system. These devices can be optimized to have high resonance frequencies (up to the MHz range) and have very low mass (10⁻¹¹ kg). Therefore, MEMS can perform fast scanning motion without exciting resonances in the mechanical loop of the SPM, and hence scan the surface without causing the image distortion from which conventional piezo scanners suffer. We have designed a MEMS z-scanner which we have integrated in commercial AFM (atomic force microscope) and STM (scanning tunneling microscope) setups. We show the first successful AFM experiments.     

Compact two-dimensional coarse-positioner for scanning probe microscopes

We report on the design and fabrication of a compact two-dimensional xy-positioner for scanning probe microscopes. This positioner uses three piezoelectric bimorphs in flexing or length-change mode by appropriate selection of electrodes and voltage polarities. One end of these bimorphs is fixed to a rectangular metal frame while on each of the free ends two sapphire disks are fixed which can slide against the polished plates of a platform movable in the xy-plane. For moving the platform by one step, the bimorphs are deformed sequentially in one mode and they are brought back to their undeformed state simultaneously. The motion of the positioner has been tested with an optical microscope and a homemade scanning tunneling microscope.     

Collection of secondary electrons in scanning electron microscopes

Collection of the secondary electrons in the scanning electron microscope was simulated and the results have been experimentally verified for two types of the objective lens and three detection systems. The aberration coefficients of both objective lenses as well as maximum axial magnetic fields in the specimen region are presented. Compared are a standard side‐attached secondary electron detector, in which only weak electrostatic and nearly no magnetic field influence the signal trajectories in the specimen vicinity, and the side‐attached (lower) and upper detectors in an immersion system with weak electrostatic but strong magnetic field penetrating towards the specimen. The collection efficiency was calculated for all three detection systems and several working distances. The ability of detectors to attract secondary electron trajectories for various initial azimuthal and polar angles was calculated, too. According to expectations, the lower detector of an immersion system collects no secondary electrons I and II emitted from the specimen and only backscattered electrons and secondary electrons III form the final image. The upper detector of the immersion system exhibits nearly 100% collection efficiency decreasing, however, with the working distance, but the topographical contrast is regrettably suppressed in its image. The collection efficiency of the standard detector is low for short working distances but increases with the same, preserving strong topographical contrast.     

New imaging modes for lenslet-array tandem scanning microscopes

The tandem scanning microscope permits confocal images to be obtained in real time and viewed directly by eye. The light budget of these instruments may be increased from a few percent to a few tens of percent by incorporating an array of microlenses so as to increase the amount of illumination light that reaches the specimen. These instruments are configured for fluorescence imaging together with laser illumination. We describe how the versatility of the instrument may be enhanced to permit the use of incoherent light sources as well as extending the imaging modes to include bright‐field reflection.     

In situ metal tip sharpening of scanning probe microscopes

This article demonstrated a new approach for fabrication and sharpening of metal tips of scanning probe microscopes. Experimentally, a metal tip was heated and melted by a focused laser light. The tip was then sharpened by a strong electric field and consolidated as the laser was turned off. With a low‐vacuum and a high‐voltage source, a 25‐µm indium‐coated platinum wire was sharpened to a tip with diameters below 50 nm. The minimal tip radius by this method is estimated to be below 1 nm. With this technique, in situ tip sharpening for SPM would be possible. SCANNING 34: 76–79, 2012. © 2011 Wiley Periodicals, Inc.     

LEO SUPRA系列热场发射扫描电子显微镜

本文介绍LEOSUPRA系列热场发射扫描电子显微镜的性能及特点 ,主要包括肖特基场发射电子源、电子光学系统以及检测器等。     

光学显微成像系统图像清晰度评价函数的对比

图像清晰度评价函数是评价各类成像系统成像质量的一个关键函数,为找到合适的图像清晰度评价算法,采用MATLAB软件对16种适用于光学显微成像系统的清晰度评价函数进行仿真,定量分析了不同算法的灵敏度、单峰性、无偏性以及运算速度。实验表明:Laplacian函数具有较高的单峰性、无偏性和灵敏度;存在高斯噪声时,Brenner函数、Tenengrad函数和基于Prewitt算子的函数以及中值滤波-离散余弦函数稳定性好;而存在椒盐噪声时,Roberts函数综合性能最优。     

Microchannel plate detector for low voltage scanning electron microscopes

A microchannel plate detector has advantages over conventional Everhart-Thornley detectors for low voltage SEM applications. A microchannel plate can provide symmetric SEM signal detection at the low beam energies needed for non-destructive examination of integrated circuits. Microchannel plate detectors are effective for both secondary electron and backscattered electron imaging. Their relatively small size and ability to be mounted directly below the final pole piece give them improved performance relative to conventional detectors in applications requiring short working distances. A significant amount of laboratory and applications experience has shown that microchannel plates are reliable and sufficiently resistant to contamination for use in high volume, production environments which require low voltage SEM imaging.     

Scintillation SE detector for variable pressure scanning electron microscopes

We present results obtained with a new scintillation detector of secondary electrons for the variable pressure scanning electron microscope. A detector design is based on the positioning of a single crystal scintillator within a scintillator chamber separated from the specimen chamber by two apertures. This solution enables us to decrease the pressure to several Pa in the scintillator chamber while the pressure in the specimen chamber reaches values of about 1000 Pa (7.5 Torr). Due to decreased pressure, we can apply a potential of the order of several kV to the scintillator, which is necessary for the detection of secondary electrons. Simultaneously, the two apertures at appropriate potentials of the order of several hundreds of volts create an electrostatic lens that allows electrons to pass from the specimen chamber to the scintillator chamber. Results indicate a promising utilization of this detector for a wide range of specimen observations.     

A high-resolution add-on in-lens attachment for scanning electron microscopes

A compact add-on objective lens for the scanning electron microscope (SEM) has been designed and tested. The lens is < 35 mm high and can be fitted on to the specimen stage as an easy-to-use attachment. Initial results show that it typically improves the spatial resolution of the SEM by a factor of three. The add-on unit is based upon a permanent magnet immersion lens design. Apart from the extra attachment to the specimen stage, the SEM with the add-on lens functions in the normal way. The in-lens unit can comfortably accommodate specimen heights up to 10 mm. The new add-on lens unit opens up the possibility of operating existing SEMs in the high-resolution in-lens mode. By using a deflector at the top of the add-on lens unit, it can also operate as a quantitative multichannel voltage contrast spectrometer, capable of recording the energy spectrum of the emitted secondary electrons. Initial experiments confirm that a significant amount of voltage contrast can be obtained.     

Construction of 35-millimeter camera adaptors for scanning electron microscopes

Described here is the construction of two 35-mm camera adaptors for attachment to the recording monitors of scanning electron microscopes (SEM). The designs are simple and readily adaptable to almost any SEM. The choice of this camera format for recording SEM images is one of convenience as well as economy and does not sacrifice micrograph quality.     

Imaging properties of high aperture multiphoton fluorescence scanning optical microscopes

A theory for multiphoton fluorescence imaging in high aperture scanning optical microscopes employing finite sized detectors is presented. The effect of polarisation of the fluorescent emission on the imaging properties of such microscopes is investigated. The lateral and axial resolutions are calculated for one-, two- and three-photon excitation of p-quaterphenyl for high and low aperture optical systems. Significant improvement in lateral resolution is found to be achieved by employing a confocal pinhole. This improvement increases with the order of the multiphoton process. Simultaneously, it is found that, when the size of the pinhole is reduced to achieve the best possible resolution, the signal-to-noise ratio is not degraded by more than 30%. The degree of optical sectioning achieved is found to improve dramatically with the use of confocal detection. For two- and three-photon excitation axial full width half-maximum improvement of 30% is predicted.     

Electron-gas interactions in the environmental scanning electron microscopes gaseous detector

The construction of high signal-to-noise, artefact-free secondary electron images in the elevated pressure conditions of an environmental SEM is a nontrivial process. The interactions of information carrying species, as well as probe beam electrons, with the chamber gas are the major reasons for such complications. In this paper, we discuss and review the present understanding of these phenomena. In addition, we outline procedures for assessing the signal-amplifying and charge-neutralising capabilities of an environmental gas. It is only with a knowledge of such parameters and an appreciation of the gas-electron collision processes that one can optimise the microscope's operating parameters. Moreover, such information enables the separation of topographic detail from artefactual features in the detected electron images.     

An on-pole-piece-tip deflector for high-resolution,low-voltage scanning electron microscopes

A proposal to enlarge the field of view of low-voltage, high-resolution scanning electron microscopes is presented in this paper. The proposal is based upon mounting primary beam deflectors onto the pole tip of the objective lens. Simulation results show that this compact arrangement of beam deflection should widen the present attainable field of view at probe resolutions < 5 nm by over an order of magnitude.     

Bridging the gap between conventional and video-speed scanning probe microscopes

A major disadvantage of scanning probe microscopy is the slow speed of image acquisition, typically less than one image per minute. This paper describes three techniques that can be used to increase the speed of a conventional scanning probe microscope by greater than one hundred times. This is achieved by the combination of high-speed vertical positioning, sinusoidal scanning, and high-speed image acquisition. These techniques are simple, low-cost, and can be applied to many conventional microscopes without significant modification. Experimental results demonstrate an increased scan rate from 1 to 200 Hz. This reduces the acquisition time for a 200×200 resolution image from 3 min to 1 s.