Center For X-ray Optics Research Articles

Coded aperture detector: an image sensor with sub 20-nm pixel resolution.

We describe the coded aperture detector, a novel image sensor based on uniformly redundant arrays (URAs) with customizable pixel size, resolution, and operating photon energy regime. In this sensor, a coded aperture is scanned laterally at the image plane of an optical system, and the transmitted intensity is measured by a photodiode. The image intensity is then digitally reconstructed using a simple convolution. We present results from a proof-of-principle optical prototype, demonstrating high-fidelity image sensing comparable to a CCD. A 20-nm half-pitch URA fabricated by the Center for X-ray Optics (CXRO) nano-fabrication laboratory is presented that is suitable for high-resolution image sensing at EUV and soft X-ray wavelengths.

EUV Extendibility: Challenges Facing EUV at 1x and beyond

EUV extendibility: challenges facing EUV at 1x and beyond Patrick P. Naulleau, 1 Christopher N. Anderson, 1 Suchit Bhattarai, 2 Andrew Neureuther 2 Center for X-ray Optics, Lawrence Berkeley National Laboratory, Berkeley, CA, USA EECS, University of California, Berkeley, CA, USA Extreme ultraviolet chemically amplified resist performance has recently been extended to the 15-nm half pitch regime, yet line-edge roughness (LER) remains far from targets. Stochastic analysis, however, shows current LER performance to be material limited rather than photon limited. Interest in contact hole printing and contact size uniformity has dramatically increased over the past few years. As with line space printing, we find contact uniformity performance to be material limi ted rather than photon limited. Nevertheless, current resist parameters would lead to the photon noise alone exceeding the uniformity requirement by the 16-nm half pitch node with conventional mask s. The use of phase shift masks is shown to provide a significant benefit. Combining phase shift masks with relatively modest improvements in resist is predicted to lead to target performance down to 12-nm half pitch and beyond. Keyword: photoresist, extreme ultraviolet, shot noise, line-edge roughness 1. Introduction Extreme ultraviolet (EUV) lithography has now entered the pilot line phase and interest in the question of extendibility to the 1x node and beyond has greatly increased. Advanced resist testing with 0.3 numerical aperture (NA) microfield exposure tools [1,2] is now focused on extremely low-k 1 configurations and development is underway for new 0.5-NA tools [3]. Chemically amplified resist performance has recently been extended to the 15-nm half pitch regime, yet line-edge roughness (LER) remains far from targets. Here we explore the importance of photon limits relative to materials limits for the current LER values. This analysis is also extended to contact critical dimension uniformity (CDU). As we push to the deep 1x regime, resist performance cannot be considered in isolation of aerial image performance. Extending EUV to the low-k 1 regime will potentially come with significant implications on aerial image performance and thus variability. When exploring variability limits, mask and system optimization should be considered. 2. Resolution status Since achieving 16-nm half pitch (HP) performance in chemically amplified (CA) resists in 2011, progress in ultimate resolution has stalled. Figure 1 shows a plot of EUV resist resolution as a function of time. Despite the lack of progress in ultimate resolution, a sensitivity improvement from 30 mJ/cm 2 to 20 mJ/cm 2 was made between 2011 and 2012, however, at the cost of LER. Figure 1. Progress in EUV chemically amplified resist resolution as a function of time.

ChemInform Abstract: Viewing Spin Structures with Soft X‐Ray Microscopy

AbstractReview: 102 refs.

A Tabletop UV Microscope

Researchers from the Colorado State University and Lawrence Berkeley National Laboratory's Center for X-ray Optics have developed a high-resolution ultraviolet microscope that fits on a tabletop. Researchers were able to get enough resolving power from a relatively small setup by providing a better alignment of the titanium-sapphire laser, the silver-cadmium pellet, and the diffraction plates. These tabletop UV lasers have the potential to serve as candidate inspection systems for future lithographic masks

Technical Report: X-ray Wave-Front Measurements and X-ray Active Optics

In 2002, under a close collaboration with the Center for X-ray Optics (CXRO), we used ALS beamline 12.0 to perform the first experimental demonstration of wave-front analysis via the Hartmann technique in the EUV spectral range.

Thorium-Based Thin Films as Highly Reflective Mirrors in the EUV

AbstractAs applications for extreme ultraviolet (EUV) radiation have been identified, the demand for better optics has also increased. Thorium and thorium oxide thin films (19 to 61 nm thick) were RF-sputtered and characterized using atomic force microscopy (AFM), spectroscopic ellipsometry, low-angle x-ray diffraction (LAXRD), x-ray photoelectron spectroscopy (XPS), and x-ray absorption near edge structure (XANES) in order to assess their capability as EUV reflectors. Their reflectance and absorption at different energies were also measured and analyzed at the Advanced Light Source in Berkeley. The reflectance of oxidized thorium is reported between 2 and 32 nm at 5, 10, and 15 degrees from grazing. The imaginary component of the complex index of refraction, β, is also reported between 12.5 and 18 nm. Thin films of thorium were found to reflect better between 6.5 and 9.4 nm at 5 degrees from grazing than all other known materials, including iridium, gold, nickel, uranium dioxide, and uranium nitride. The measured reflectance does not coincide with reflectance curves calculated from the Center for X-Ray Optics (CXRO) atomic scattering factor data. We observe large energy shifts of up to 20 eV, suggesting the need for better film characterization and possibly an update of the tabulated optical constants.

Comparison of extreme ultraviolet reflectance measurements

The semiconductor industry has pushed linewidths on integrated-circuit chips down to 100 nm. To pattern ever finer lines by use of photolithography, the industry is now preparing the transition to extreme ultraviolet lithography (EUVL) at 13 nm by 2007. As EUVL matures, the requirements for the accuracy of reflectivity and wavelength measurements are becoming tighter. A high absolute accuracy and worldwide traceability of reflectance measurements are mandatory for worldwide system development. A direct comparison of EUV reflectance measurements at the Advanced Light Source (ALS) Center for X-Ray Optics (CXRO) and Physikalisch-Technische Bundesanstalt (PTB) yield perfect agreement within the mutual relative uncertainties of 0.14% for reflectance and 0.014% for wavelength.

Popular Berkeley lab X-ray data booklet reissued

X-ray scientists and synchrotron-radiation users who have been patiently waiting for an updated version of the popular X-Ray Data Booklet last published in 1986 by the Center for X-Ray Optics at the Lawrence Berkeley National Laboratory can breathe a sigh of relief. The venerable ''little orange book'' has now been reissued under the auspices of CXRO and the Advanced Light Source (ALS) with an April printing of 10,000 paper copies and the posting of a Web edition at http://xdb.lbl.gov.

Soft X-ray spectromicroscopy for in situ study of corrosion

Corrosion of filings from an ASTM Grade 60 steel reinforcing bar was examined in situ using the soft X-ray transmission microscope XM-1, operated by the Center for X-ray Optics and located at the Advanced Light Source at E.O. Lawrence Berkeley National Laboratory. Iron distribution maps generated from X-ray images of a steel filing before and after the addition of water show the change in elemental iron distribution in the sample. The decreased presence of iron in the distribution map after the introduction of water indicates the oxidation of iron — the anodic reaction in steel corrosion. These results demonstrate the utility of transmission soft X-ray microscopy for in situ corrosion studies.

Use of soft X-ray microscopy for analysis of early-stage biofilm formation

Soft X-ray microscopy (SXM) using synchrotron radiation is a newly-available technology for high resolution imaging of hydrated specimens, making it potentially valuable for the study of biofilms. In this research, SXM was used to investigate bacterial spatial distribution and biofilm structure during the early stages of colonization of an exposed surface. Pseudomonas putida DMP-1 formed a closely-packed monolayer on a silicon nitride substratum after 24 hours of growth. The initial development of a multilayer, interlocking structure was observed. The soft X-ray images were taken with the XM-1 transmission X-ray microscope, located at the Center for X-ray Optics, Lawrence Berkeley National Laboratory. The results indicate the potential of SXM to provide new insights for the study of biofilms.

Use of soft X-ray microscopy for analysis of early-stage biofilm formation

Soft X-ray microscopy (SXM) using synchrotron radiation is a newly-available technology for high resolution Imaging of hydrated specimens, making it potentially valuable for the study of biofilms. In this research, SXM was used to investigate bacterial spatial distribution and biofilm structure during the early stages of colonization of an exposed surface. Pseudomonas putida DMP-1 formed a closely-packed monolayer on a silicon nitride substratum after 24 hours of growth. The initial development of a multilayer, interlocking structure was observed. The soft X-ray images were taken with the XM-1 transmission X-ray microscope, located at the Center for X-ray Optics, Lawrence Berkeley National Laboratory. The results indicate the potential of SXM to provide new insights for the study of biofilms.

High Spatial Resolution Soft X-Ray Microscopy and Microanalysis of Thick and Hydrated Materials

Abstract The Center for X-ray Optics (CXRO) built and operates a high-resolution soft x-ray microscope (XM-1) at the Advanced Light Source in Berkeley. We report on the use of this instrument in a variety of scientific fields, including biology, civil engineering and environmental sciences. The microscope is a conventional (full field) x-ray microscope, which uses zone plate lenses to provide high resolution transmission images. The optical setup is similar to the Göttingen x-ray microscope, operated at the BESSY synchrotron radiation facility in Berlin, Germany. A condenser zone plate, fabricated by the Göttingen group, is illuminating the sample and an objective zone plate, fabricated by Erik Anderson (CXRO), is forming an enlarged image on an x-ray CCD camera. While the optical path of the microscope is in vacuum, the sample is at atmospheric pressure, flushed by helium. The spatial resolution of our microscope is 43 nm, measured as the distance from 10%-90% intensity in the image of a knife-edge.

Imaging of ASR Gel by Soft X-Ray Microscopy

The soft x-ray transmission microscope XM-1 was used to examine alkali-silicate reaction (ASR) gel morphology in an experimental investigation of the alkali-aggregate reaction (AAR). The XM-1 microscope is operated by the Center for X-ray Optics on beamline 6.1 of the Advanced Light Source, a third generation synchrotron radiation facility operated by the Ernest O. Lawrence Berkeley National Laboratory. The instrument is unique as samples can be observed wet, with high resolution (43 nm), over time, as chemical reactions proceed. Soft x-ray microscopy was used to examine the in situ reaction of ground ASR gel, obtained from a large dam, and solutions of sodium hydroxide, calcium hydroxide, and combined sodium and calcium hydroxide. From this investigation, it appears that ASR gel combines with alkalis present in pore solution to produce a reaction gel capable of swelling, while the reaction of the ASR gel in the presence of calcium hydroxide and no alkalis results in the formation of a structure resembling C-S-H. It is theorized that the formation of C-S-H or a related compound will decrease the degree of swelling that would otherwise result from the formation of an alkali-aggregate reaction product. The C-S-H-like structure may also contribute strength. These hypotheses are currently under investigation.

Interferometry using undulator sources (invited, abstract)

Optical systems for extreme ultraviolet (EUV) lithography need to use optical components with subnanometer surface figure error tolerances to achieve diffraction-limited performance [M.D. Himel, in Soft X-Ray Projection Lithography, A.M. Hawryluk and R.H. Stulen, eds. (OSA, Washington, D.C., 1993), 18, 1089, and D. Attwood et al., Appl. Opt. 32, 7022 (1993)]. Also, multilayer-coated optics require at-wavelength wavefront measurement to characterize phase effects that cannot be measured by conventional optical interferometry. Furthermore, EUV optical systems will additionally require final testing and alignment at the operational wavelength for adjustment and reduction of the cumulative optical surface errors. Therefore, at-wavelength interferometric measurement of EUV optics will be the necessary metrology tool for the successful development of optics for EUV lithography. An EUV point diffraction interferometer (PDI) has been developed at the Center for X-Ray Optics (CXRO) and has been already in operation for a year [K. Goldberg et al., in Extreme Ultra Lithography, D.T. Attwood and F. Zernike, eds. (OSA, Washington, D.C., 1994), K. Goldberg et al., Proc. SPIE 2437, to be published, and K. Goldberg et al., J. Vac. Sci. Technol. B 13, 2923 (1995)] using an undulator radiation source and coherent optics beamline at the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory. An overview of the PDI interferometer and some EUV wavefront measurements obtained with this instrument will be presented. In addition, future developments planned for EUV interferometry at CXRO towards the measurement of actual EUV lithography optics will be shown.

Precision manufacturing using LIGA (abstract)

Our objective is the fabrication of small high-precision parts using LIGA, which can be used in a variety of industrial applications. LIGA is a combination of deep x-ray lithography, electroplating, and replication processes that enables the fabrication of microstructures with vertical dimensions several millimeters high, lateral dimensions in the micrometer range, and submicron tolerances. On beamline 10.3.2, at the Advanced Light Source (ALS), the Center for X-ray Optics (CXRO) has built an end station suitable for LIGA. The ALS is an excellent source of radiation for this application. The CXRO, in close collaboration with Sandia National Laboratory and the Jet Propulsion Laboratory, has developed the other essential process steps of mask making, resist development, x-ray exposure, and electroplating. This technology provides a powerful tool for mass production and miniaturization of mechanical systems into a dimensional regime not accessible by traditional manufacturing operations. We will present several applications that exploit the characteristics of the LIGA process: the fabrication of magnetic laminations for a high precision stepping motor; miniature octopole lens for advanced e-beam lithography; high-aspect-ratio x-ray collimating grids for astronomy; and microscopic tumblers for nuclear security.

Use of capillary optics as a beam intensifier for a Compton x-ray source.

The use of Kumakhov capillary optics will significantly enhance the performance of near-monochromatic, Compton backscattered x-ray programs. The Vanderbilt University Medical Free-Electron Laser Center is developing the capability to create these tunable x rays for medical imaging. The present transport has only reflection optics, and the beam is quite large in diameter at the laboratory. Low loss collimation of this beam would allow higher x-ray intensities after transport. This article describes experimental and computer simulation results which predict the expected performance for a multifiber Kumakhov collimator for use in the x-ray beam transport. Estimates from our research are that a multifiber optic formed of individual polycapillary fibers could be used to capture the full 7 mrad of the Vanderbilt x-ray beam and collimate it to a 1-2 mrad divergence with approximately 40%-50% transmission efficiency. This optic should increase the x-ray intensity at the laboratory level by a factor of > or = 5 by decreasing the beam divergence and subsequent spot size. Additionally, analysis of monolithic optics of fused multicapillary fibers predicts an increase in the intensity of the x rays at the laboratory by a factor of 55. These optics can have tapered channels that greatly decrease their exit divergence. This will greatly enhance the capabilities of this unique x-ray source. This article reports the initial results from a collaboration between Vanderbilt, The Center for X-Ray Optics at University at Albany, SUNY, and X-Ray Optical Systems in Albany, NY.

The kumakhov lens: Prospects for x-ray microanalysis

A new optical system based on multiple reflection of X-rays and neutrons along specially shaped solid surfaces first proposed by Kumakhov has undergone intense and rapid development in the USSR and more recently in a joint development effort between the Institute for Roentgen Optical Systems (IROS) in Moscow, Russia, and the Center for X-Ray Optics (CXO) in Albany, NY. The most used X-ray Optical System (XOS) utilizes thin hollow microcapillary tubes arranged appropriately to capture X-rays from a divergent source to produce a quasiparallel beam or a focussed beam as shown schematically in Figure 1. Alternatively, a parallel beam of X-rays (or neutrons) can be deflected or focused. It is also possible to use such systems for energy filtering or energy selection.The first such systems were constructed from single hollow capillaries and showed that X-rays could indeed be captured and focussed as shown in Figure 2. These "first generation" Kumakhov optics are most efficient for relatively low energy X-rays (0.5-2 keV).

Recent developments in SHADOW

SHADOW is the ray-tracing program of choice for the synchrotron radiation community. The program can raytrace essentially any x-ray optical system and predict image shapes and intensities with great accuracy. Several new extensions have been included. The optical database has been extended to the 10-eV region by inclusion of the new data from the Center for X-ray Optics, Lawrence Berkeley Lab. Two major physical models have been added to the code; the first being a surface roughness model to add to the error surface option. The second addition is to the crystal model. It now includes an asymmetric crystal, a model of a mosaic crystal, and a model for the Johannson case of a figured, bent crystal. Until now, SHADOW has operated under Digital Equipment Corporation’s (DEC) VMS operating system (VMS). The advent of powerful RISC workstations has prompted a porting to the UNIX environment, opening a multiplatform operability. So far, the code has been validated for Ultrix (DEC) and is undergoing beta testing for Sun operating system and IBM Corporation’s AIX operating system. Porting to a 386/486 platform is also considered. Finally, an optimization algorithm based on a combination of the Simplex and simulated annealing is being implemented.

Detection of crystalline phase in cross-section multilayer thin film by High-Resolution Transmission Electron Microscopy

High-resolution transmission electron microscopy has proven to be very useful in direct detection of crystalline phases that exist over extremely small volumes, yielding information about structure, orientation, and, under appropriate circumstances, composition. In this paper, we report the detection of a crystalline phase in the tungsten-rich layer of an annealed 7 nm-period tungsten-carbon multilayer produced at the Center for X-Ray Optics at the Lawrence Berkeley Laboratory.The multilayers were prepared by dc magnetron sputtering at floating temperature. The argon sputter gas pressure was 0.0020 torr. Different techniques were employed to produce cross-section and plan-view samples for TEM. For cross-section samples, 70 bilayers of W and C were sputtered on semiconductor-grade Si (111) wafers. For plan-view samples, the substrates on which the multilayer was grown consisted of 3 mm-diameter 300-mesh copper microscope grids, mounted on glass slide with Crystalbond® vacuum adhesive. After a deposition of 4 bilayers of W-C, keeping the same sputtering parameters as those of the Si substrates to guarantee the same layer thicknesses, the glass slide was soaked in acetone to disolve the Crystalbond®, leaving the multilayer spanning the holes of the copper grids. Both the Si-substrate and copper-grid samples were annealed at 500°C for 4 hours under vacuum of 10−6 torr. The annealed Si-substrate sample was then prepared for cross-section by mechanical grinding, and ion milling in a cold stage at 5kV. The cross-section sample was studied in a JEOL JEM 200CX with ultrahigh resolution goniometer, with the eletron beam parallel to the [112] of the Si substrate. The plan-view sample was studied in a Philips 301 operating at 100kV.

Scanning transmission x-ray microscopy of cultured cells

Soft x-rays provide an interesting possibility for imaging thick specimens at resolution better than that of the light microscope. Because of the way these x-rays interact with matter, a transmission image is formed essentially entirely by absorption, and suffers negligible blurring due to scattering. The only serious effect of sample thickness is to attenuate the x-ray beam. At x-ray wavelengths between 2.3 and 4.4 nm, the absorption is particularly appropriate for examining thick biological specimens. In this region water is weakly absorbing while carbon is strongly absorbing. As a result, absorption by a whole live cell is dominated by biological molecules. The magnitude of the absorption is such that cells up to ten microns thick can be imaged. Finally, these x-rays can traverse up to a millimater of air without serious attenuation. Transmission microscopy of live cells in air is thus possible.The technology for producing high resolution x-ray images has only recently become available. Zone-plate focussing has been perfected to the point where an x-ray beam spot 40 nm in diameter can be formed. This spot can be raster-scanned over a specimen and the transmitted x-rays detected, to form a scanning transmission x-ray microscope (STXM) image. The spot size, and resolution, are expected to improve to about 20 nm in the next few years. A very intense source of nearly monochromatic soft x-rays is also needed, at present only available at synchrotron light sources. We are working with a group from SUNY Stony Brook, IBM Watson Laboratories, and the Center for X-Ray Optics who have just finished building a microscope at the National Synchrotron Light Source. Two other microscopes are now being built, at Daresbury in England, and in Berlin.