Inspection of a Spherical Triple VLS-Grating for Self-Seeding of FLASH at DESY

Abstract

Abstract. To take benefit from the improved brilliance of the laser-like source, proposed beamlines at Free Electron Lasers (FEL) require optical elements of excellent precision, characterised by slope errors beyond the state of the art limit of 0.5urad rms for plane and spherical shape. Part of the monochromator beamline for self-seeding at the vacuumultraviolet Free Electron Laser (FLASH) at DESY is a triple Variable Line Spacing (VLS) grating of spherical shape. The three grating structures on a common substrate will cover the wavelength range from 6.4 to 60nm. The challenging specifications of these grating structures are characterised by a slope error of less than 0.25urad rms and very stringent parameters for the VLS-polynomial [1]. These grating structures have been measured by use of the Nano Optic Measuring Machine (NOM) at BESSY. Based on the principle of deflectometry the BESSY-NOM [2,3] represents the latest generation of slope measuring metrology instruments. The NOM enables the inspection of optical components with a measurement uncertainty in the range of 0.05urad rms. This is a five to tenfold improvement compared to state of the art metrology tools of today. Here it is demonstrated how these outstanding metrology capabilities have been applied for a sound characterisation of a challenging precision optical component with error limits five times below the specifications. In the shown example the grating's figure accuracy has been characterised by linescans and surface mapping measurements of the optical active sections. In additional measurements under Littrow condition the higher order diffraction signals of the laser pencil beam have been traced to measure the groove density variation of the different grating-structures with a lateral resolution of 1mm. In contrast to the sparse and point like measurements of the manufacturer, these high resolution measurements yield a "slope deviation equivalent" resulting from imperfections in the line density variation.

Towards super-resolution interference microscopy metrology of x-ray variable-line-spacing diffraction gratings: recent developments

“…For a long time, the preferred tool for grating characterization at synchrotron radiation facilities [4,5,[13][14][15] has been surface-slope-measuring long trace profilers (LTPs), arranged in the Littrow configuration for one dimensional (1D) measurements of the diffraction angle variation. In many cases, LTP metrology is capable of measuring groove density distributions with the required accuracy, but, due to low spatial resolution, fails to measure groove phase coherence, which determines the beamline spectral resolution achievable with the grating.…”

Investigation on lateral resolution of surface slope profilers

“…The LTP-II optical sensor is based on the pencil beam interferometer, initially suggested and patented by K. Von Bieren in 1985 [24][25][26] and firstly applied in long trace profilers for precision characterization of x-ray mirror surface slope topography in 1986 [27,28]. Having a number of important modifications to the optical schematic [29][30][31], LTP type profilers remain one of only two classes of surface slope measuring tools that are broadly in use at metrology laboratories of x-ray facilities [14][15][16][17][32][33][34][35][36][37][38][39][40][41][42][43].…”

Wavefront Sensing for Evaluation of Extreme Ultraviolet Microscopy

“…WavePropaGator [ 34 ] was used to simulate the wavefront propagation of the beam ( Figure 8 ); the simulation was highly precise. The residual slopes and the height profile of the mirrors were all measured at the Helmholtz Zentrum Berlin by F. Siewert et al before installing them in the FLASH beamlines [ 35 , 36 ]. The profiles of the two plane mirrors in the tunnel were included in the wavefront simulation, as well as the correct distances between optical elements along the beamline.…”