Angelic L. Pearson

High resolution protein localization using soft X-ray microscopy.

Soft X‐ray microscopes can be used to examine whole, hydrated cells up to 10 µm thick and produce images approaching 30 nm resolution. Since cells are imaged in the X‐ray transmissive ‘water window’, where organic material absorbs approximately an order of magnitude more strongly than water, chemical contrast enhancement agents are not required to view the distribution of cellular structures. Although living specimens cannot be examined, cells can be rapidly frozen at a precise moment in time and examined in a cryostage, revealing information that most closely approximates that in live cells. In this study, we used a transmission X‐ray microscope at photon energies just below the oxygen edge (λ = 2.4 nm) to examine rapidly frozen mouse 3T3 cells and obtained excellent cellular morphology at better than 50 nm lateral resolution. These specimens are extremely stable, enabling multiple exposures with virtually no detectable damage to cell structures. We also show that silver‐enhanced, immunogold labelling can be used to localize both cytoplasmic and nuclear proteins in whole, hydrated mammary epithelial cells at better than 50 nm resolution. The future use of X‐ray tomography, along with improved zone plate lenses, will enable collection of better resolution (approaching 30 nm), three‐dimensional information on the distribution of proteins in cells.

20-nm-resolution Soft x-ray microscopy demonstrated by use of multilayer test structures

A spatial resolution of 20 nm is demonstrated at 2.07-nm wavelength by use of a soft x-ray microscope based on Fresnel zone plate lenses and partially coherent illumination. Nanostructural test patterns, formed by sputtered multilayer coatings and transmission electron microscopy thinning techniques, provide clear experimental results.

Dynamical x-ray microscopy investigation of electromigration in passivated inlaid Cu interconnect structures

Quantitative time-resolved x-ray microscopy mass transport studies of the early stages of electromigration in an inlaid Cu line/via structure were performed with about 40 nm lateral resolution. The image sequences show that void formation is a highly dynamic process, with voids being observed to nucleate and grow within the Cu via and migrate towards the via sidewall. Correlation of the real time x-ray microscopy images with postmortem high voltage electron micrographs of the sample indicates that the void nucleation occurs at the site of grain boundaries in Cu, and that the voids migrate along these grain boundaries during electromigration.

Soft x-ray microscopy to 25 nm with applications to biology and magnetic materials

We report both technical advances in soft X-ray microscopy (XRM) and applications furthered by these advances. With new zone plate lenses we record test pattern features with good modulation to 25 nm and smaller. In combination with fast cryofixation, sub-cellular images show very fine detail previously seen only in electron microscopy, but seen here in thick, hydrated, and unstained samples. The magnetic domain structure is studied at high spatial resolution with X-ray magnetic circular dichroism (X-MCD) as a huge element-specific magnetic contrast mechanism, occurring e.g. at the L2,3 edges of transition metals. It can be used to distinguish between in-plane and out-of-plane contributions by tilting the sample. As XRM is a photon based technique, the magnetic images can be obtained in unlimited varying external magnetic fields. The images discussed have been obtained at the XM-1 soft X-ray microscope on beamline 6.1 at the Advanced Light Source in Berkeley. # 2001 Elsevier Science B.V. All rights reserved.

Demonstration of 20 nm half-pitch spatial resolution with soft x-ray microscopy

The full field, transmission soft x-ray microscope XM-1 is a valuable imaging instrument for many scientific and technological areas involving nanometer features. Operating from 300 to 1800 eV, it combines high spatial resolution, elemental discrimination, magnetic sensitivity, and a capability of imaging in various experimental conditions, such as with applied magnetic fields and electric currents. In this article, we report experiments that enable accurate spatial resolution measurement, using a new type of test pattern, made from thinned multilayer coatings. The resolution of the microscope was measured to be 20 nm, using this method.

Electromigration in passivated Cu interconnects studied by transmission x-ray microscopy

Time-resolved x-ray microscopy studies of the electromigration in inlaid Cu line/via structures were performed on focused-ion-beam-prepared cross sections of an advanced interconnect layer system. Multiple x-ray images were recorded at 1.8 keV photon energy while stressing the passivated Cu structures with an applied current. The image sequences show that void formation is a dynamic process, with voids being observed to nucleate and grow within the Cu via and migrate towards the via sidewall. Correlation of the real time x-ray microscopy images with postmortem high voltage transmission electron and scanning electron micrographs indicates that the void nucleation occurs at grain boundaries in the copper, and that the voids migrate along these grain boundaries during electromigration. By taking multiple images at different viewing angles, the three-dimensional arrangement of an interconnect stack with Cu line / via structures was reconstructed. In future studies time-resolved tomography will be used to visu...

A full field transmission X-ray microscope as a tool for high-resolution magnetic imaging

The XM-1 soft X-ray microscope is a user-dedicated facility located at the Advanced Light Source at Lawrence Berkeley National Laboratory and has recently been established as a tool for high-resolution imaging of magnetic domains. It is a conventional full field transmission microscope which is able to achieve a resolution of 25 nm by using high-precision zone plates. It uses off-axis bend-magnet radiation to illuminate samples with elliptically polarized light. When the illumination energy is tuned to absorption edges of specific elements, it can be used as an element-specific probe of magnetism on the 25 nm scale with contrast provided by magnetic circular dichroism. The illumination energy can be adjusted between 250-850 eV. This allows magnetic imaging of elements including chromium, iron and cobalt. The spectral resolution has been shown to be E//spl Delta/E=500-700. This spectral resolution allows a high sensitivity so that magnetization has been imaged within layers as thin as 3 nm. Since this is a photon based magnetic microscopy, fields can be applied to the sample even during imaging without affecting the spatial resolution. The current system can apply in-plane or out-of-plane fields of a few kOe.

Electromigration in integrated circuit interconnects studied by X-ray microscopy

Abstract To study mass transport phenomena in advanced microelectronic devices with X-rays requires penetration of dielectric and Si layers up to 30 μm thick. X-ray imaging at 1.8 keV photon energy provides a high amplitude contrast between Cu or Al interconnects and dielectric layers and can penetrate through the required thickness. To perform X-ray microscopy at 1.8 keV, a new Ru/Si multilayer was designed for the transmission X-ray microscope XM-1 installed at the Advanced Light Source in Berkeley. The mass flow in a passivated Cu interconnect was studied at current densities up to 107 A/cm2. In addition, we demonstrated the high material contrast from different elements in integrated circuits with a resolution of about 40 nm.

High-resolution soft x-ray microscopy

The XM-1 is a soft x-ray full-field microscope that uses zone plates for the condenser and objective lenses. One of the main features of XM-1 is the high spatial resolution, which is made possible by the fine features of the objective zone plate. At present, the microscope uses a zone plate with an outer zone width of 25 nm. Several test patterns containing periodic lines and spaces were fabricated to measure the resolution of the microscope. Experimental data shows that the microscope can resolved 25 nm features. As simulations indicate that good contrast can be observed with even smaller features, test patterns with finer features are being fabricated to actually determine the resolution limit of the microscope.

Experimental analysis of high-resolution soft x-ray microscopy

The soft x-ray, full-field microscope XM-1 at Lawrence Berkeley National Laboratorys (LBNL) Advanced Light Source has already demonstrated its capability to resolve 25-nm features. The soft x-ray, full-field microscope XM-1 at Lawrence Berkeley National Laboratorys (LBNL) Advanced Light Source has already demonstrated its capability to resolve 25-nm features. This was accomplished using a micro zone plate (MZP) with an outer zone width of 25 nm. Limited by the aspect ratio of the resist used in the fabrication, the gold-plating thickness of that zone plate is around 40 nm. However, some applications, in particular, biological imaging, prefer improved efficiency, which can be achieved by high-aspect-ratio zone plates. We accomplish this by using a bilayer-resist process in the zone plate fabrication. As our first attempt, a 40-nm-outer-zone-width MZP with a nickel-plating thickness of 150 nm (aspect ratio of 4:1) was successfully fabricated. Relative to the 25-nm MZP, this zone plate is ten times more efficient. Using this high-efficiency MZP, a line test pattern with half period of 30 nm is resolved by the microscope at photon energy of 500 eV. Furthermore, with a new multilayer mirror, the XM-1 can now perform imaging up to 1.8 keV. An image of a line test pattern with half period of 40 nm has a measured modulation of 90%. The image was taken at 1.77 keV with the high-efficiency MZP with an outer zone width of 35 nm and a nickel-plating thickness of 180 nm (aspect ratio of 5:1). XM-1 provides a gateway to high-resolution imaging at high energy. To measure frequency response of the XM-1, a partially annealed gold island pattern was chosen as a test object. After comparison with the SEM image of the pattern, the microscope has the measured cutoff of 19 nm, close to the theoretical one of 17 nm. The normalized frequency response, which is the ratio of the power density of the soft x-ray image to that of the SEM image, is shown in this paper.