Multifunctional light beam source for surface slope measuring long trace profilers

Abstract

To fully exploit the advantages of fourth-generation synchrotron light sources, diffraction-limited-storage-rings (DLSR) and fully coherent free electron lasers (FELs), beamline mirrors and diffraction grating must be of exceptional quality. To achieve the required mirror and grating quality, the metrology instrumentation and methods used to characterize these challenging optics and, even more so, optical assemblies must also offer exceptional functionality and performance. One of the most widely used slope measuring instruments for characterizing x-ray optics is the long trace profiler (LTP). The easily reconfigurable mechanical design of the LTP allows optimization of the profiler arrangement to the specifics of a particular metrology task. Here, we discuss the optical schematic, design, and performance of an original multifunctional light beam source that provides functional flexibility of the LTP optical sensor. With this source, the LTP can be easily reconfigured for measurements of x-ray mirrors or diffraction gratings that have widely different source coherence requirements. Usage of a source with a low degree of coherence for mirror metrology helps to suppress the LTP systematic errors due to spurious interference effects in the LTP optical elements. A high-coherence narrow-band source is used for groove-density-distribution characterization of x-ray diffraction gratings. The systematic error and spatial resolution of the LTP with the different sources is also measured and analyzed.

Characterization of groove density variation of VLS gratings with ALS XROL LTP-II in different operation modes

“…However, the difference of different modes in the effective lateral resolution can hardly explain the observed significant difference of the low-spatial-frequency variations of the groove density distributions, extracted from the same measurement data, but processed with different algorithms. We can speculate that the observed difference may also relate to a difference in the systematic error and spatial resolution of the LTP-II in the different operation/data processing modes [3] (see also discussion in Ref. [11]).…”

“…The major observation from the LTP-II measurements with the grating is the strong correlation between the LTP-II mode of operation and the residual (after subtraction of the best-fit 1 st -or 3 rd -order polynomial functions) groove density variation. The smaller residual variation corresponds to the operation modes the SGB/GMF and PBI/SOPF with lower spatial resolution [3,10].…”

“…In Ref. [3], we investigate the dependence of the angular systematic error on the operation/data processing mode of the LTP-II using simulations with the optical model of the LTP under development [16]. In conclusion, we can use the observed variations of the parameters of the polynomial function, best-fitting the groove density distributions extracted from the measurement data, obtained with the LTP-II in different modes to estimate the current achievable accuracy of measurements of the corresponding parameters (note that the current version of the LTP-II is not capable to measure the 0 g parameter) as These estimations can be compared with the accuracy achieved in the ZYGO NewView˗7300 microscope measurements with the 300-lines/mm VLS diffraction gratings, developed for the ALS MAESTRO beamline: The results of these measurements are discussed in detail in Ref.…”

“…We have recently demonstrated that the LTP spatial resolution of surface slope measurements in these two modes is significantly different [10]. Additionally, these modes of operation can have different sensitivity to the LTP-II systematic errors [3,11]. Therefore, it is interesting to understand the effect of the different resolution on the groove measurements with the LTP in the SGB and PBI modes of operation.…”

“…The long trace profiler, LTP-II [1], available at the Advanced Light Source (ALS) X-ray Optics Laboratory (XROL) [2], was recently upgraded by replacing a multimode diode laser light source with a single-mode, wavelength-stabilized, fiber-coupled diode laser system [3]. The upgrade has enabled us to reliably characterize the lateral variation of groove density of variable-line-spacing (VLS) x-ray diffraction gratings in the manner discussed, for example, in Refs.…”