Peter Z. Takacs

Optical constants for thin films of C, diamond, Al, Si, and CVD SiC from 24 Å to 1216 Å

A method for deriving optical constants from reflectance vs angle of incidence measurements using a nonlinear least-squares curve-fitting technique based on the chi(2) test of fit is presented and used to derive optical constants for several thin-film materials. The curve-fitting technique incorporates independently measured values for the film surface roughness, film thickness, and incident beam polarization. The technique also provides a direct method for estimating probable errors in the derived optical constants. Data are presented from 24 A to 1216 A for thin-film samples of C, synthetic diamond, Al, Si, and CVD SiC. Auger electron spectroscopy depth profiling measurements were performed on some of the samples to characterize sample composition including oxidation and contamination.

The penta‐prism LTP: A long‐trace‐profiler with stationary optical head and moving penta prisma)

Metrology requirements for optical components for third generation synchrotron sources are taxing the state‐of‐the‐art in manufacturing technology. We have investigated a number of effect sources in a commercial figure measurement instrument, the Long Trace Profiler II (LTP II), and have demonstrated that, with some simple modifications, we can significantly reduce the effect of error sources and improve the accuracy and reliability of the measurement. By keeping the optical head stationary and moving a penta prism along the translation stage, the stability of the optical system is greatly improved, and the remaining error signals can be corrected by a simple reference beam subtraction. We illustrate the performance of the modified system by investigating the distortion produced by gravity on a typical synchrotron mirror and demonstrate the repeatability of the instrument despite relaxed tolerances on the translation stage.

Versatile spectrometer for experiments using synchrotron radiation at wave-lengths greater than 100 nm

Abstract Most applications of synchrotron radiation (SR) have been in the extreme ultraviolet and X-ray spectral domains, i.e. wave-lengths less than 100 nm. In the spectral region longward of 100 nm, SR may also be superior to other sources for certain experiments. To date, SR above 100 nm has been exploited most extensively by fluorescence lifetime measurements. Experiments such as circular dichroism, magnetic circular dichroism, various emission spectroscopies and photoacoustic spectroscopy can also use to advantage the high intensity, continuous tunability, short term stability and polarization provided by SR. We have constructed a versatile spectrometer capable of performing the experiments mentioned above and suitable for use to wavelengths less than 130 nm. It will be operated at the SURF II (NS) ring pending completion of the NSLS at Brookhaven National Laboratory. In addition, we describe a modular computer system which will be used to control the operation of and collect and process spectral data from this spectrometer system.

Optimal estimation of finish parameters

This paper discusses basic issues involved in the estimation of surface spectra from laboratory measurements, the development of physically-based spectral models, and the estimation of finish parameters and their associated errors.

Long Trace Profile Measurements On Cylindrical Aspheres

A new long-trace optical profiling instrument is now in operation at Brookhaven National Laboratory measuring surface figure and macro-roughness on large optical components, principally long cylindrical mirrors for use in synchrotron radiation beam lines. The non-contact measurement technique is based upon a pencil-beam interferometer system. The optical head is mounted on a linear air bearing slide and has a free travel range of nearly one meter. We are able to sample surface spatial periods between 1 mm (the laser beam diameter) and 1 m. The input slope data is converted to surface height by a Fourier filtering technique which distributes the random noise error contributions evenly over the entire trace length. A number of optical components have been measured with the instrument. Results are presented for fused silica cylinders 900 mm and 600 mm in length and for a fused silica toroid and several electroless nickel-plated paraboloids.

Improvements in the accuracy and the repeatability of long trace profiler measurements

Modifications of the long trace profiler at the Advanced Photon Source at Argonne National Laboratory have significantly improved its accuracy and repeatability for measuring the figure of large flat and long-radius mirrors. Use of a Dove prism in the reference beam path corrects phasing problems between mechanical errors and thermally induced system errors. A single reference correction now completely removes both of these error signals from the measured surface profile. The addition of a precision air conditioner keeps the temperature in the metrology enclosure constant to within +/-0.1 degrees C over a 24-h period and has significantly improved the stability and the repeatability of the measurements. Long-radius surface curvatures can now be measured absolutely with a high degree of confidence. These improved capabilities are illustrated with a series of measurements of a 500-mm-long mirror with a 5-km radius of curvature. The standard deviation in the average of ten slope profile scans is 0.3 microrad, and the corresponding standard deviation in the height error is 4.6 nm.

Specification of surface figure and finish in terms of system performance

We describe methods of predicting the degradation of the performance of a simple imaging system in terms of the statistics of the shape errors of the focusing element and, conversely, of specifying those statistics in terms of requirements on image quality. Results are illustrated for normal-incidence, x-ray mirrors with figure errors plus conventional and/or fractal finish errors. It is emphasized that the imaging properties of a surface with fractal errors are well behaved even though fractal-power spectra diverge at low spatial frequencies.

Future metrology needs for synchrotron radiation grazing-incidence optics

Abstract An International Workshop on Metrology for X-ray and Neutron Optics, the first of its kind, was held on March 16–17, 2000, at the Advanced Photon Source at Argonne National Laboratory. Metrology specialists, beamline engineers and scientists, and vendors from around the world met to evaluate current metrology instrumentation and methods used to characterize the surface figure and finish off long grazing-incidence optics used in synchrotron radiation beamlines, and to consider future needs for synchrotron, free-electron laser, and neutron sources. This paper summarizes the discussions on mirror and metrology requirements for the current and next-generation X-ray sources. Some recommended strategies for the needs of the future are also given.

Simultaneous measurement of absorption and circular dichroism in a synchrotron spectrometer

Abstract We describe a method for simultaneously measuring the absorption spectrum and circular dicroism (CD) of a sample in a spectrometer that employs a transparent window photomultiplier detector. During the spectral scan, the computer control system simultaneously monitors the CD signal, the high voltage applied to the detector and the current in the storage ring. A servo-control system maintains the detector current at a constant value during the scan by varying the applied high voltage. The logarithm of the photomultiplier gain is calculated from the value of the high voltage using the predetermined characteristics of the tube. The gain value is corrected for the decrease of the storage ring current with time. During a single spectral scan, the computer generates a pseudo-absorption spectrum, which is the sum of the absorbance of the sample and of various wavelength-dependent system parameters. An absolute absorption spectrum is produced when the pseudo-absorption spectrum of a “blank” sample is subtracted from the desired sample spectrum. Absorption spectra measured by this method agree with spectra obtained with a double-beam spectrophotometer.

Using a straightness reference in obtaining more accurate surface profiles from a long trace profiler

The long trace profiler (Takacs et al.) has found significant applications in measuring the surfaces of synchrotron optics. However, requirements of small slope errors at all spatial wavelengths of the synchrotron optics mandate more accurate slope measurements. A straightness reference for the long trace profiler greatly increases the accuracy of the instrument. Methods of using the straightness reference by interpreting the sequential interference patterns are discussed and results of measurements are presented.