Focused Probe Beam Research Articles

CW coherent detection lidar for micro-Doppler sensing and raster-scan imaging of drones.

We demonstrate a continuous-wave (CW) coherent detection lidar (CDL) capable of detecting micro-Doppler (propeller) signatures and acquiring raster-scan images of small unmanned aerial systems/vehicles (UAS/UAV). The system uses a narrow-linewidth 1550 nm CW laser and takes advantage of mature and low-cost fiber-optics components from the telecommunications industry. Using either collimated or focused probe beam geometry, lidar based detection of characteristic periodic motions of drone propellers up to a remote distance of 500 m has been achieved. Furthermore, by raster scanning a focused CDL beam with a galvo-resonant mirror beamscanner, two-dimensional images of flying UAVs up to 70 m range were obtained. Each pixel of the raster-scan images provides both lidar return signal amplitude and target radial speed information. The raster-scan images obtained up to 5 frames per second make it possible to discriminate various UAV types based on their profile and even resolve the presence of payloads. With feasible improvements, the anti-drone lidar is a promising alternative to expensive EO/IR and active SWIR cameras used in counter-UAV systems.

High resolution spatially extended 1D laser scattering diagnostics using volume Bragg grating notch filters.

Laser light scattering systems with volume Bragg grating (VBG) filters, which act as spectral/angular filters, have often been used as a point measurement technique, with spatial resolution as low as a few hundred μm, defined by the beam waist. In this work, we demonstrate how VBG filters can be leveraged for spatially resolved measurements with several μm resolution over a few millimeters along the beam propagation axis. The rejection ring, as determined by the angular acceptance criteria of the filter, is derived analytically, and the use of the ring for 1D laser line rejection is explained. For the example cases presented,i.e., for a focused probe beam waist with a diameter of ∼150 μm, the rejection ring can provide resolution up to several millimeter length along the beam propagation axis for a 1D measurement, which is also tunable. Additionally, methods to further extend the measurable region are proposed and demonstrated, using a collimation lens with a different focal length or using multiple VBG filters. The latter case can minimize the scattering signal loss, without the tradeoff of the solid angle. Such use of multiple VBGs is to extend the measurable region along the beam axis, which differs from the commonly known application of multiple filters, to improve the suppression of elastic interferences. 1D rotational Raman and Thomson scattering measurements are carried out on pulsed and DC discharges to verify this method. The system features compactness, simple implementation, high throughput, and flexibility, to accommodate various experimental conditions.

Estimation of the Thickness Profile of a Human Skull Phantom by Ultrasound Methods Using a Two-Dimensional Array

The paper presents the results of evaluating the thickness profile of a skull phantom using a two-dimensional ultrasound array consisting of piezoelectric elements with a center frequency of 2.1 MHz. Two pulse-echo ultrasound methods were used in the experiment: the A-mode elementwise measurements and scanning with a focused probing beam created by the entire array using delay-and-sum (DAS) beamforming. The obtained thickness profiles are compared with the reference thickness profile obtained using X-ray computed tomography. It was shown that ultrasound DAS beamforming with a focused probing beam makes it technically possible to estimate the thickness profile of the skull phantom.

Quantitative Cryo-Scanning Transmission Electron Microscopy of Biological Materials.

Electron tomography provides a detailed view into the 3D structure of biological cells and tissues. Physical fixation by vitrification of the aqueous medium provides the most faithful preservation of biological specimens in the native, fully hydrated state. Cryo-microscopy is challenging, however, because of the sensitivity to electron irradiation and due to the weak electron scattering of organic material. Tomography is even more challenging because of the dependence on multiple exposures of the same area. Tomographic imaging is typically performed in wide-field transmission electron microscopy (TEM) mode with phase contrast generated by defocus. Scanning transmission electron microscopy (STEM) is an alternative mode based on detection of scattering from a focused probe beam, without imaging optics following the specimen. While careful configuration of the illumination and detectors is required to generate useful contrast, STEM circumvents the major restrictions of phase contrast TEM to very thin specimens and provides a signal that is more simply interpreted in terms of local composition and density. STEM has gained popularity in recent years for materials science. The extension of STEM to cryomicroscopy and tomography of cells and macromolecules is summarized herein.

Correlation function of spin noise due to atomic diffusion

We use paramagnetic Faraday rotation to study spin noise spectrum from unpolarized Rb vapor in a tightly focused probe beam in the presence of N$_2$ buffer gas. We derive an analytical form for the diffusion component of the spin noise time-correlation function in a Gaussian probe beam. We also obtain analytical forms for the frequency spectrum of the spin noise in the limit of a tightly focused or a collimated Gaussian beam in the presence of diffusion. In particular, we find that in a tightly focused probe beam the spectral lineshape can be independent of the buffer gas pressure. Experimentally, we find good agreement between the calculated and measured spin noise spectra for N$_2$ gas pressures ranging from 56 to 820 torr.

Amplitude and phase beam shaping for highest sensitivity in sidewall angle detection

In integrated circuits manufacturing, specific structures are used as tools to evaluate the quality of the lithographic process, and the shape of these structures is often described by a few parameters, of which in particular the sidewall angle suffers from considerable inaccuracies. Using scalar diffraction theory, we investigate whether a properly shaped cylindrically focused probing beam could increase the ability to detect tiny changes in this angle in the case of a cliff-like structure, modeled as a phase object. This paper describes the theoretical formulation used to calculate the optimized beam and compares its performance with the case of a focused plane wave.

(Keynote) Metrology for Nanoscale Complex Semiconductor Systems

Pushing the limits in IC-technology towards the nanometer scale, led to the development of complex systems and 3D-devices (like Finfets, TFET, nanowires) whereby novel materials and in particular interfacial interactions play a crucial role. These increasing technological challenges have boosted the importance of metrology and created a demand for concepts suited to probe very small volumes and enable atomic scale observations. One obvious solution is then to abandon 1D-metrology completely and to focus on metrology such as Atom probe tomography (APT) which is an extremely powerful method providing composition analysis within very small volumes (a few nm3) with high sensitivity and accuracy. Due to its excellent spatial and depth resolution (in many cases with the ability to resolve lattice planes and individual atoms) alloy composition analysis in small trenches, unexpected in diffusion in such volumes, dopant distribution and dopants decorating defects can in principle be identified. Nevertheless the presence of many materials with different evaporation fields does induce severe artefacts and trajectory aberrations which cannot always be corrected, causing severe measurement inaccuracies. Complementary to the resolving power of APT, is the application of scanning probes (spm, ssrm, c-afm) which enables to grasp the electrical activity of dopants or identify conduction paths within such volumes. As SPM is inherently a 2D-method, concepts for expanding into the depth dimension are explored (cfr Scalpel SPM, ion beam sputtering icw SPM,..). Applications in logic device engineering, failure analysis and memory cell development (RRAM, CBRAM, NAND) demonstrate the power of this approach. Moreover “standard” solutions like STM to image crystal structure and defects at the surface may need to be modified in view of the very close proximity of insulating regions.Despite their unique 3D-resolving power, APT and SPM suffer from a poor productivity and a lack of statistical averaging over large areas as required in more production oriented metrology. A solution can be found through ensemble measurements whereby spatial resolution is provided by the device under investigation and not by the probing beam. We will illustrate this concept through applications of “self focusing SIMS” which allows to determine the composition from trenches (SiGe, InGaAs,..) as small as 20 nm without having an ion beam with nm-resolution. Similarly crystallinity of III-V growth in narrow trenches (< 50 nm) can be obtained through channeling RBS whereby again we use a large beam but nevertheless probe the information from an array of very fine features. In all these cases, the averaging over a large array provides excellent statistics and in some cases dramatically improved productivity through the enhanced signal versus the case of a very focused probe beam. The latter is ultimately exemplified in Raman experiments on narrow SiGe-trenches where we demonstrate that the signal from very narrow features (20 nm) is significantly enhanced (50-100x) as compared to it blanket counterpart enabling to probe composition and structural properties from a small volume.

Transient absorption microscopy: advances in chemical imaging of photoinduced dynamics

AbstractThis article reviews the state of the art of ultrafast transient absorption microscopy, discusses current experimental concepts and highlights future challenges. The advantages of transient absorption microscopy over other micro‐spectroscopic techniques are its high optical resolution combined with high temporal resolution as well as its ability to study non‐fluorescent and weakly fluorescent molecular species and to probe excited‐state processes. In conventional transient absorption spectroscopy the spectroscopic information usually presents a spatial average over the focal spot of the typically weakly focused probe beam. Transient absorption microscopy, however, enables investigations of the excited state dynamics in individual microscopic areas of a sample. Hence, the technique does not only yield detailed morphological information based on a label‐free molecular contrast, but also gives insight into the ultrafast morphology‐dependent photoinduced processes in heterogeneous samples. Different variations of transient absorption microscopy have found a number of applications ranging from material sciences to biology, which are discussed in this review together with different setup modifications and approaches towards transient absorption spectroscopy with spatial resolution below the diffraction limit. image

Droplet streams for serial crystallography of proteins

Serial diffraction of proteins requires an injection method to deliver protein molecules - preferably uncharged, fully hydrated, spatially oriented, and with high flux - into the crossed beams of an alignment laser and a focused probe beam of electrons or X-rays of typically only a few tens of microns diameter. The aim of this work has been to examine several potential droplet sources as to their suitability for this task. We compare Rayleigh droplet sources, electrospray sources, nebulizers and aerojet-focused droplet sources using time-resolved optical images of the droplet beams. Shrinkage of droplets by evaporation as a means of removing most of the water surrounding the proteins is discussed. Experimental measurements of droplet size, conformation of proteins after passing through a Rayleigh jet, and triboelectric charging are presented and conclusions are drawn about the source configuration for serial diffraction. First experimental X-ray diffraction patterns from Rayleigh droplet beams doped with 100nm gold balls are shown.

Fourier-Transform Diffuse Reflection IR-Spectroscopy in Studies of Thermal Oxidation of X18H10T Steel

Methodology potentialities of the Fourier-transform diffuse reflection IR-spectroscopy with sharply focused probing beam are discussed as applied to studies of X18H10T steel gaseous oxidation at 100–800°C. The method is shown to be promising as regards investigating thermodynamics of the process.

Information-Optical 3D-Techniques for Corrosion Monitoring

Methodical basis of using information-optical 3D-techniques and computer optical video microprobes in a high-performance monitoring of the thickness, topography, and statistical properties of thin corrosion layers on microscopic surface areas of a metal or semiconductor is considered. Forming a microscopic video image of such an area of an electrode and measuring the intensity distribution of the reflected optical signal in its field, one can obtain a 3D relief-map of the thickness and other properties of islets, new phase nuclei, and secondarily deposited layers on the electrode area in real time. The possibilities of combining the scanning and reflectometric 3D video microprobe techniques, diffuse-reflection infrared spectroscopy with a sharply focused probing beam, as well as photo-EMF technique, ellipsometry, and electrochemical methods when studying the corrosion behavior of metals and alloys, enable one to monitor the nonuniformity of the thickness and chemical composition of the corrosion layers. The information-optical 3D techniques of treating the images provides highly informative monitoring of the nonuniformity degree of the thickness, defectiveness, and chemical composition of the layer on the whole studied microscopic surface area of a corroded metal.

Four-dimensional microscopy of defects in integrated circuits

We demonstrate four-dimensional microscopy of defects in integrated circuits by a technique that combines laser-scanning confocal reflectance microscopy with one-photon optical-beam-induced current (1P-OBIC) imaging. Accurate information is obtained about the three-dimensional structure of the defect and the kind of material (metal, semiconductor, or dielectric) that is damaged by the defect. The same focused probe beam simultaneously produces the 1P-OBIC and reflectance signals from the illuminated sample spot. The hardware development cost is minimal for a laser-scanning confocal microscope, and the image reconstruction procedure is computationally efficient. Imaging is demonstrated on defects that are caused by electrical overstress and unwanted generation centers. Exclusive three-dimensional distributions of the semiconductor and metal sites in the integrated circuit reveal defect features that are difficult to recognize with confocal or 1P-OBIC imaging alone.

Scanner 3D-probe and infrared probe spectroscopy for investigating macrodefects of thermooxide layers

A possibility of combining the techniques of scanner reflectometric 3D-microprobe and diffuse-reflection infrared spectroscopy (with a focused probing beam) for investigating the nonuniformity of the thickness and chemical composition of oxide coatings on metals and alloys is considered. As an example, the morphology and composition of a nonuniform layer obtained at the surface of a X18H10 chromium steel after the oxidation in air at a temperature of 500°C are studied. Having properly digitally treated the optical images of a small probed surface area we can introduce and determine those parameters that affect the protective properties of a morphologically nonuniform surface thermooxide layer. The diffuse-reflection probing infrared spectroscopy makes it possible to determine and control the composition of a nonuniform oxide layer at selected probed areas.

Spatial speckle correlometry in applications to tissue structure monitoring

Asymptotic behavior of temporal autocorrelation functions of speckle intensity fluctuations induced by tissue scanning with a focused probe beam is experimentally studied for the transition from a single-scattering to a multiple-scattering mode. Such parameters as the exponential factor (or the Hurst coefficient) and the Hausdo dimension are proposed for the characterization and the visualization of the variations of the studied tissues' optical properties in generalized form. We studied reversible transition between various scattering modes stimulated by the application of certain chemical agents to the human sclera samples using speckle intensity correlation analysis; corresponding results are presented. The possibilities of the scattering structures imaging with local estimations of the exponential factor of speckle intensity fluctuations are shown in in vitro experiments with samples of human skin epidermis.

Confocal filtering of the instantaneous image in scanned darkfield alignment

Darkfield (DF) alignment signals are commonly obtained by scanning targets beneath a focused probe beam while integrating the light scattered outside a pupil stop, the latter serving to block specular reflections. Additional information pertaining to target position can be obtained by refocusing the collected light to form an instantaneous DF image that moves across the detector as the target is scanned through the focused illumination probe. A simple method to achieve a significantly sharper and more selective response to the target is to apply a confocal prefiltering to the instantaneous image before integrating it to read out the signal level, i.e., successive frames of the instantaneous image are multiplied by a template aperture corresponding to the image of a centered alignment mark. A fiducial scan is used to approximately center the template, providing a reference position. In the case of a test wafer so grainy that the conventional DF signal is significantly broadened and almost overwhelmed by random scatter, the signal obtained with a confocal filter modified to trigger on target symmetry and matched edge slopes is sharper by an order of magnitude and is virtually free of background.

Multiphoton photochemical and collisional effects during oxygen-atom flame detection

A Nd:YAG-pumped dye-laser system was used to two-photon excite oxygen atoms at 225.6 nm in an atmosphericpressure CH(4)-N(2)O-N(2) flame. Subsequent emission at 844.7 nm from the directly populated state as well as a stronger emission at 777.5 nm that was due to the O(3p(3)P ? 3p(5)P) collisional-energy transfer process was monitored. Two-photon-resonant oxygen-atom and hydrogen-atom (656.3-nm) emissions were also observed in the absence of a flame. Closer examination revealed that the tightly focused probe beam was producing these atoms by promoting multiphoton photolysis of the oxidizer as well as of the fuel molecules. Thus this type of laser-diagnostic probe is potentially quite intrusive, depending on the combustion region that is probed as well as on the laser energies used.

Contactless electrical testing of large area specimens using electron beams

An e-beam test system has been developed to detect opens and shorts in three-dimensional networks of conductors embedded in a dielectric matrix. The specimens, which measure up to 100×100 mm, are tested electrically without making physical contact and without mechanical stepping. The system comprise two flood beams and a focused probe beam deflected over the full specimen area at a resolution of better than 3000 lines at 1–2 kV beam energy. One flood gun is located at the rear, the other at the front of the specimen. The change in secondary emission imposed by impinging electrons charging or discharging the surface is detected as voltage contrast which allows one to discriminate between uninterrupted and interrupted as well as shorted pairs of conductors. The physical concept, the contrast mechanism, and the electron optics of the system will be presented together with experimental results.