July 7, 2004

NSLS 2004 Annual Users’ Meeting Workshop

Advanced Optical Systems and Metrology for High Power and Coherent Beam Lines

Future light source beam line instrumentation will present challenges to users and instrument designers to take full advantage of the brightness and coherence of the x-rays delivered by the new sources. Since Brookhaven National Laboratory has a strong tradition in the development of metrology instrumentation in support of synchrotron radiation optics, it is appropriate that this session on optical systems and metrology reviewed the current state-of-the-art in optical technology and metrology, and explored avenues that may lead to improved capabilities for the future.

Chris Jacobsen from Stony Brook University, who has extensive experience in the generation and use of coherent x-rays with existing sources, provided a summary of capabilities of various groups around the world to produce small spot sizes from present sources of coherent x-rays. A Japanese group at SPring8 has demonstrated 90nm spot sizes with Kirkpatrick-Baez focusing optics, and 15nm line spacing in zone plates have been reported by workers at the Center for X-ray Optics at Berkeley.

Following Jacobsen’s introduction to coherent imaging at x-ray wavelengths, the workshop heard from two speakers who are primarily involved with normal incidence UV and visible optics for large programs at Lawrence Livermore National Laboratory and at NASA Marshall Space Flight Center. The surface figure and finish requirements on normal incidence optics for DUV (Deep Ultraviolet) and EUV (Extreme Ultraviolet) lithography and for large space telescopes are nearly the same as those for grazing incidence optics, so it was appropriate to hear from John S. Taylor of LLNL and from Phil Stahl of NASA about mirror fabrication and metrology technologies that are being supported by these projects. Taylor discussed the development of diffraction-limited multilayer-coated optics for normal incidence EUV lithography systems that have been developed over the past several years by the Virtual National Laboratory consortium of 3 national laboratories and 3 companies involved in developing tools for the semiconductor manufacturing industry. Slope errors on these optical components are in the 200 nrad range, which is where synchrotron optics will need to be for effective utilization of the next generation of high brightness x-ray sources. Stahl gave an overview of future NASA space telescope missions and the technology that will be required to build large, lightweight mirrors and structures. Over the past few years he has organized a symposium, “NASA Tech Days”, for NASA contractors and university grant recipients to present progress in technology development programs funded by government agencies. Of interest to the SR community are the various materials development projects investigating the use of Be, SiC, and other ceramic and composite materials as high power mirror substrates, and the fabrication technologies that accompany these new materials.

Don Golini from QED Technologies in Rochester, NY, gave a presentation on a new surface finishing technique that looks quite promising as an alternative to conventional pitch polishing for the production of spherical and aspherical surfaces. His company has developed various polishing machines that use magnetorheological fluids as the polishing medium. These fluids change viscosity by orders of magnitude when a magnetic field is applied and can conform to exotic surface shapes not possible with hard laps, such as ellipsoids and toroids. A great deal of research has gone into characterizing the polishing process, which is highly deterministic. With the appropriate metrology, it is possible to quickly correct for figure errors and converge rapidly to the desired surface shape.

Refractive optics for focusing x-ray beams are a new addition to the techniques available to users for producing small spot sizes. Ken Evans-Lutterodt (National Synchrotron Light Source) gave an overview of the subject and discussed the production of the optimum elliptical hole shape with an e-beam writer and the use of diamond as a substrate for high heat load situations. Ali Khounsary, Argonne National Laboratory, followed with a discussion of high heat load optics. He emphasized the need for a systematic approach to the design of optical systems that incorporates knowledge of the source parameters, mirror substrate characteristics, mechanical design, and testing.

Following the lunch break, Peter Takacs (BNL/Instrumentation) discussed metrology issues related to the Long Trace Profiler (LTP). The LTP is widely used to measure the surface quality of large aspheric x-ray optics. In order to reach the 100 nrad measurement accuracy level, improvements are needed in the quality of the internal optical components in the system. Custom made glass prisms with superpolished surfaces will be needed to reduce the systematic error to below the current 1 µrad level. Wayne McKinney (Advanced Light Source, Lawrence Berkeley Laboratory) followed with a discussion of issues that are driving the direction of SR metrology. Although the LTP has served well in the past, other alternative metrology techniques need to be explored, such as stitching interferometry, Hartmann sensors, and the multi-beam autocollimator techniques such as those proposed by Polack’s group at SOLEIL in France and by Weingärtner’s group at the PTB in Braunschweig, Germany.

The final session of the day began with a description of Kirkpatrick-Baez (KB) mirror nanofocusing systems by Gene Ice from Oak Ridge National Laboratory. The practical limit to the focal spot size is in the range of 10 to 30 nm, which can be produced by highly-demagnifying (~1000:1) elliptical cylinders. These mirrors need to have slope error tolerances of less than 200 nrad in order to perform correctly. The technique of differential deposition, which he has pioneered, appears to be a promising tool for converting cylindrical surfaces into accurate elliptical shapes.

The final two talks were presented jointly by Riccardo Signorato (ACCEL, France) and Daniel Hausermann (Advanced Photon Source, Argonne National Laboratory) as they related to the fabrication and use of modular piezoelectric bimorph mirrors as adaptive optics. The piezoelectric bimorph has been developed by Signorato and is currently in use at a number of sites, including beam lines at the APS. Hausermann uses a pair of bimorphs in a KB configuration to focus a 500µm x 500µm hard x-ray beam down to a 10 µm spot. The longest size currently available is 1050 mm in length with N=33 actuator segments. Signorato is looking into adding a Shack-Hartmann wavefront analyzer to the system to enable automated closed-loop focus control for the mirror.

Following the final talk, a quick tour of the Optical Metrology Laboratory in the Instrumentation Division was offered to those participants who remained. The workshop was successful in bringing together members of several different communities, including conventional optics fabricators, end users, and metrology specialists, to share thoughts and ideas on the current state of optical component technology and to chart a course toward future capabilities.

ACKNOWLEDGEMENTS
This work was sponsored (in part) by the U.S. Department of Energy under Contract No. DE-AC02-98CH10886.

FOR MORE INFORMATION
Peter Z. Takacs (lead author)
Building 535B
Instrumentation Division
Brookhaven National Laboratory
Upton, New York
11973-5000
Tel.: (631) 344-2824
Fax: (631) 344-5773
Email: takacs@bnl.gov

Steven Hulbert
Building 725D
National Synchrotron Light Source
Brookhaven National Laboratory
Upton, New York
11973-5000
Tel.: (631) 344-7570
Fax: (631) 344-3238
Email: hulbert1@bnl.gov