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ALSNews           Vol.11           January 31, 1995

Table of Contents


    1. BEAMLINE BREAKTHROUGHS
    2. SCINTILLATING SUBJECTS
    3. TIMELY TOPICS

1. BEAMLINE BREAKTHROUGHS

** SITE-SPECIFIC PHOTOELECTRON DIFFRACTION PROBES MAGNETIC INTERFACES **
(contact: csfadley@lbl.gov)

Last fall, researchers at Beamline 9.3.2 began running experiments on their advanced photoelectron spectrometer/diffractometer endstation, which includes a Scienta ES200 analyzer. This system has high energy resolution (energy spread/energy = 1/10,000) and receives high-intensity x rays from its bend-magnet source. A group led by Zahid Hussain (ALS), Charles Fadley (U. of California-Davis and LBL Materials Sciences Division), and David A. Shirley (Penn State U.) used the new beamline and endstation to study the structures of surface atoms on tungsten, and of buried interface atoms between a tungsten substrate and an overlayer of the ferromagnet gadolinium. The choice of tungsten and gadolinium samples reflects an interest in the growth of magnetic thin films on non-magnetic substrates, an important subject in the development of the next generation of magnetic storage technologies.

The group analyzed the (110) surface of tungsten, for which the 4f core level of the clean surface exhibits a binding-energy shift of 320 meV between bulk and surface peaks. The resolutions of these peaks were the best recorded to date, and the photoelectron diffraction patterns of the bulk and surface components were measured over the full hemisphere above the surface for the first time. The resulting data have already provided a rigorous test of the multiple scattering theory used to model photoelectron diffraction and to determine surface structures from experiment; this theory and related experimental methods are of interest to several groups at the ALS.

After depositing a monolayer of gadolinium on the tungsten surface, the experimenters found that the tungsten 4f spectra revealed an even larger binding-energy shift between bulk tungsten atoms and tungsten atoms at the tungsten-gadolinium interface. This effect occurs because the gadolinium overlayer acts as a dielectric screen, decreasing the binding energy of the interface electrons. The group also measured the photoelectron diffraction patterns of these interface atoms for the first time, a development which holds considerable promise for determining the structural properties of buried interfaces.

Beamline 9.3.2 has a spherical grating monochromator designed to operate in the energy range 30-1500 eV. The beamline is currently being modified to provide circularly polarized as well as linearly polarized radiation, and to allow rotation of the analyzer in the horizontal plane.

2. SCINTILLATING SUBJECTS

** NEW FACES **
(contact: yang-mo_koo@macmail.lbl.gov)

Yang-Mo Koo is a recent arrival at the ALS who will be visiting here until the end of 1995. Koo's home is in South Korea, where he is the Magnet Group Leader at Pohang Light Source and an associate professor of Materials Science at Pohang University of Science and Technology. He is working with Howard Padmore of the ALS Experimental Systems Group, developing the technique of total reflection x-ray fluorescence (TXRF) at Beamline 10.3.2, and with a research team led by Piero Pianetta that has pioneered synchrotron-radiation-based TXRF at Stanford Synchrotron Radiation Laboratory (SSRL). TXRF is a surface-sensitive technique (see description in following article) which Koo and Padmore are using to look for trace elements on silicon wafers.

** ADDRESSING SEMICONDUCTOR INDUSTRY NEEDS WITH TRACE-ELEMENT DETECTION ADVANCES **
(contact: howard_padmore@macmail.lbl.gov)

Total-reflection X-Ray Fluorescence (TXRF), a technique for measuring trace contaminants on surfaces, is widely used by the semiconductor industry for quality control in manufacturing processes. Decreasing scale in integrated circuit (IC) manufacture demands lower levels of metal surface contaminants, which impair IC function. Improved wafer cleaning and handling can accomplish the needed decreases in contamination, but they must be complemented by measurement techniques for ultra-trace amounts of contaminants on the silicon surface. These techniques must resolve elemental species, must be surface-sensitive, and must detect extremely small amounts of contaminants: 10e9 atoms/sq. cm (one millionth of the number of atoms required to form a single atomic layer of contaminant) is the current state of the art using high-power rotating-anode x-ray sources. The semiconductor industries roadmap indicates that by the end of this century, detection methods sensitive to <10e7 atoms/sq. cm will be required.

TXRF most commonly involves irradiating a sample with x rays and detecting the resulting fluorescence x rays with an energy-resolving (to resolve elemental species) detector. Irradiating the sample surface at grazing incidence, so that x rays typically penetrate only 20 angstroms into the surface, makes TXRF surface-specific. This leaves the problem of extreme sensitivity, now being addressed by synchrotron-based TXRF experiments at SSRL, and soon at the ALS. Synchrotron radiation's advantages for TXRF are its high flux (photons per second) and the linear polarization of synchrotron x rays in the horizontal plane. Performing reflection and detection in the horizontal plane of polarization significantly reduces the background due to x-ray scattering from the silicon surface, so that signals due to surface contaminants can be detected with greater sensitivity. The SSRL team, led by Piero Pianetta, has achieved sensitivities of 10e8 atoms/sq. cm on a multipole wiggler beamline, and development of a high-count-rate detector should place the 10e7 atoms/sq. cm detection limit within reach. Calculations show that if the ALS is run at 1.9 GeV electron energy (vs. SSRL's 3.0 GeV), an ALS bend-magnet source could equal the useful flux on the sample from the SSRL multipole wiggler. This is because horizontal brightness, not flux, is the figure of merit for TXRF. The ALS' smaller horizontal source size in comparison to SSRL thus makes up for the flux advantages of an SSRL multipole wiggler over an ALS bend magnet.

Developing TXRF to the required sensitivity at both SSRL and ALS meets another semiconductor industry requirement: year-round access to TXRF capabilities. By coordinating shutdown dates, ALS and SSRL will be able to provide TXRF continuously. TXRF demonstration experiments at the ALS will begin in April at Beamline 10.3.2 (shared with deep-etch x-ray lithography). It is hoped that this demonstration will lead to a fully optimized beamline for TXRF studies at the ALS.

3. TIMELY TOPICS

** NEW ALS EH&S PROGRAM MANAGER **

Georgeanna Perdue has been selected as the new ALS Environment, Health, and Safety Program Manager, a position for which she has been "acting-manager" for several months. Perdue is a key player in the ALS' commitment to provide a safe and efficient working environment and she encourages users to ask questions about safety issues and hazards. She, with the help of other ALS staff members, completed the ALS safety video and ALS Safety Handbook that have become key parts of the ALS user training program, designed to give an overview of safety policies and procedures at the ALS.