Wisconsin Synchrotron Radiation Center Research Articles

Alignment techniques for calibration and installation of a 6-m Toroidal Grating Monochromator

The University of Wisconsin Synchrotron Radiation Center Optics Group has recently completed the installation of three 6-m Toroidal Grating Monochromator (TGM) beamlines on the Aladdin ring. Two of these beamlines image bending-magnet radiation and the third images radiation from the 30-period undulator on loan to the Synchrotron Radiation Center from the Stanford Synchrotron Radiation Laboratory and Lawrence Berkeley Laboratory. Each beamline required an accurate alignment strategy. This is due to the long source-to-final-image distance and the five independent critical elements: entrance mirror, entrance slit, grating chamber, exit slit, and exit mirror. The TGM grating chamber houses up to six gratings on a revolving carousel which is scanned with a sine-drive. We will describe grating stability checks and adjustments to linearize the grating scan calibration. We will also outline the basic alignment procedures from the source point, through the grating chamber, to the final focus. The procedure we have used provided sufficiently accurate alignment to realize the high throughput and resolution capabilities of the 6-m TGMs. The methods also allow later checks on critical degrees of freedom of the optical components.

New undulator and conventional lines at the Wisconsin Synchrotron Radiation Center (invited)

We describe the recent, intensive construction program of new beamlines at the Wisconsin Synchrotron Radiation Center. In particular, we discuss technical features of new TGM lines, and the development of the advanced photoemission spectromicroscope MAXIMUM, based on undulator radiation.

6-m TGM implementation at the Wisconsin Synchrotron Radiation Center (SRC)

We recently commissioned the SRC/Vanderbilt University 6-m toroidal grating monochromator beamline. The problems with mirror heating, signal normalization, monochromator control, and scattered light reduction have led to several innovations on this line, making for a better user interface and adding to the stability and reliability of the final image. We discuss these innovations, and how they affect the user. They include a built-in laser alignment system, a sapphire windowed gate valve, entrance mirror temperature stabilization, computer automation and control, a beam-chopper, and the capability of real-time monitoring of the photon flux during the experiment. We have used noble gas resonance lines to carefully characterize the wavelength resolution of the beamline as a function of energy over a range of beamline parameters. The results demonstrate that by limiting the horizontal width of the image at the slits, and by masking outer portions of the gratings, the flux/bandwidth ratio can be improved at the points where the exit slit location does not coincide with the Rowland circle. These same resonance lines, in conjunction with accurate measurements of the grating rotation angle as a function of scan-drive displacement, are used to correct for imperfections in the surface of the drive-flat and thus obtain a precise and accurate determination of wavelength versus scan-drive displacement.

A simple monochromator based on x-ray multilayer mirrors

A simple two-mirror monochromator based on multilayer films has been developed as a part of a reflectometer under construction at the University of Wisconsin Synchrotron Radiation Center. Measurements of the performance of this monochromator and the characterization of a typical component mirror have been made. At AlKα the monochromator has a peak transmission of 9%, which is 55% of the theoretical value. Its resolution, λ/Δλ, is 35.

Status report on X-ray lithography

The need to use X-ray lithography for the large-scale manufacture of future generations of integrated circuits is discussed and an overview of world efforts to develop the necessary technologies is presented. The emphasis is on synchrotron radiation from electron storage rings as the light source. The work of the Wisconsin Center for X-Ray Lithography at the University of Wisconsin Synchrotron Radiation Center is discussed, including recent results in the areas of exposure stations, X-ray resists, masks and computer-aided process modeling.

A synchrotron radiation x-ray lithography beam line of novel design

The new synchrotron radiation x-ray lithography facility of the University of Wisconsin is presented and discussed in detail. The facility will make use of the electron storage ring Aladdin, at the Univeristy of Wisconsin Synchrotron Radiation Center. The optical design of the beam line allows large field exposures (5×5 cm2) and a tunable bandpass spectral distribution in the range 300–5000 eV. This is achieved by a suitable combination of mirrors for the low-pass part of the filter and of absorption materials for the high-pass filters. Radiation intensity is such to allow exposures of the order of 10 s for PMMA with the beam line operated at the widest setting. The expected power and spectral distribution uniformity at the wafer is better than 2%.

Observation of nonuniform energy band shifts for Ge on GaAs(110)

Fractional monolayer studies of Ge on GaAs(110) were performed with angle-resolved photoemission at the Wisconsin Synchrotron Radiation Center. Normal emission data for 14 < hν <27 eV were analyzed. The most significant effects of the Ge adatoms are shifts in photoemission peaks known to be due to GaAs interband transitions. The shifts, which are nonuniform, are due to specific nonuniform shifts in interband transitions. Valence band (VB) shifts relative to the VB maximum can be determined from some of the transitions. The top VB shifts downward and the next deeper VB shifts upward in the neighborhood of one-third the distance from Γ to X on the Σ line (0.57 Å −1). We suggest the shifts are the result of chemisorption-induced charge transfers whose resonance-like states decay over several atomic layers below the surface.

Observation of Ge-induced electronic states at the Ge:GaAs(110) interface by means of polarization-dependent UPS

Strong evidence for Ge-induced valence band states for ∠0.5 monolayer (ML) Ge on cleaved GaAs(110) is presented. Normal emission energy distribution curves have been measured for two different polarization [A⊥ mirror plane (MP) and A∥MP] for photon energies from hν = 14–27 eV at the University of Wisconsin Synchrotron Radiation Center. Four different kinds of Ge-induced features are observed: (i) attenuation of clean substrate transitions, (ii) energy shifts of substrate transitions, (iii) new transitions with hν dispersion, and (iv) new transition with no dispersion. The latter dispersionless transition observed for hν between 17 and 23 eV must originate from a two- dimensional structure, and we propose that it is due to a Ge:GaAs(110) interface state. With respect to the valence band maximum, its binding energy (EB) is 6.8 eV which compares to EB = 7.95 eV predicted for a coverage of 3 ML Ge by Mazur, Pollmann, and Schmeits for a Ge–Ga interface state. The observed interface state shows a relatively strong polarization dependence which implies that the initial state symmetry is odd. Strong Ge-induced transitions are observed with binding energies between 1 and 3 eV which show large dispersion. This means that the initial states must have a three-dimensional character. They are identified as GaAs(110) substrate transitions whose matrix elements are strongly enhanced by the chemisorption of Ge, making them observable in UPS.

Angle-resolved photoelectron spectroscopy of CO2 with synchrotron radiation

The angular asymmetry parameter β has been determined for the first four bands in the photoelectron spectrum of CO2 as a function of photon energy: X 2Πg(15–50 eV), A 2Πu(19–28 eV), B 2Σu+(19–28 eV), and C 2Σg+(21–67 eV). Use is made of a monochromatized beam of photons from the Wisconsin Synchrotron Radiation Center. The experimental results are compared with theoretical calculations employing the multiple scattering method and the overall agreement is good. In addition, the theoretical predictions for the effect on β of shape resonances have been verified except that the experimental widths are broader. This broadening has been partially accounted for in recent calculations of Swanson et al. by including the effects of vibrational motion. Finally, sharp changes in β as a function of vibrational level have been noted for the second and fourth electronic bands. Such behavior has been found for a variety of photon energies, and the results are examined against prevailing theory.

Angular distribution in the photoelectron spectrum of the ground and first excited vibrational bands of the X 2Σg+ state in N2+ measured as a function of photon energy

Angle-resolved photoelectron spectra were taken of the first ionization state of N2 as a function of photon energy using a monochromatized beam of polarized photons from the Wisconsin Synchrotron Radiation Center. From these results the angular parameters for the first two vibrational states were determined over a range of photon energies from 17.7 to 31.0 eV. In energy ranges where resonance absorption, accompanied by autoionization, is expected to occur, β for the first excited vibrational level (v1) was frequently characterized by near zero values. In energy regions free of discrete autoionization v1 was found to have a value of β considerably larger (∼0.4 to 0.8 units) than that for the ground vibrational level (v0). This result is shown to be in accordance with recent calculations by Dehmer et al., based on the effect of vibrational motion on β in the vicinity of a shape resonance.

Symmetry determination of surface states on GaAs (110) using polarization-dependent, angle-resolved photoemission

New occupied surface states are observed on the (110) cleavage face of GaAs. Using the polarized character of the radiation at the Wisconsin Synchrotron Radiation Center the symmetry of the states in the mirror plane are determined. Among current calculations of the electronic structure, only that which includes bond length relaxation yields results that are consistent with the observations.

Recent instrumentation developments at the university of Wisconsin synchrotron radiation center

Two new monochromators have been installed at the Wisconsin Synchrotron Radiation Center and four more are being designed and constructed for installation later this year. The new instruments installed are an uhv version of a vertically dispersing Seya-Namioka monochromator and an adjustable slit version of the “Grasshopper” grazing incidence monochromator built at the University of Wisconsin Physical Sciences Laboratory for use at the Stanford Synchrotron Radiation Project The new instruments to be installed later are a 4 normal incidence monochromator designed around a high efficiency Bausch and Lomb grating ruled for the Sky Lab Program and three grazing incidence toroidal grating monochromators designed for use with special Jobin-Yvon holographic gratings. The work on the toroidal grating monochromator is proceeding in collaboration with research groups from the IBM Watson Laboratories and the University of Pennsylvania. Operating characteristics achieved or to be expected with these instruments will be discussed.

Optical constants of nickel, iron, and nickel-iron alloys in the vacuum ultraviolet

Optical constants were obtained by Kramers-Kronig analysis of normal-incidence reflectivities of Ni, Fe, concentrated FeNi alloys, and ordered and disordered ${\mathrm{Ni}}_{3}$Fe. Measurements were made from 2 to 27 eV and extended to 35 eV in Ni. Specimens were either thin evaporated films or bulk specimens that were argon-ion sputtered and annealed in ultrahigh vacuum. Data were taken in a mini-computer-controlled vacuum ultraviolet reflectometer at the Wisconsin Synchrotron Radiation Center. The results are described in terms of recent electron energy-band calculations for Ni and Fe and features in the optical conductivity are interpreted in terms of the coherent-potential-approximation theory. Computations of the electron energy-loss function are presented without discussion.