Reflectometer with laser-plasma source and Czerny-Turner monochromator

General description

This reflectometer with a laser-plasma source and a Czerny-Turner monochromator is designed for research work on studying various spectra in the radiation range of 5-50 nm, as well as for studying phenomena caused by X-ray radiation in the same wavelength range. In particular, the main tasks of operation are related to the certification of X-ray optical elements (multilayer X-ray mirrors, multilayer thin-film filters). The reflectometer can be used at temperatures of 18-30℃ and relative humidity up to 80%.

External appearance of a reflectometer with a laser-plasma source and a Czerny-Turner monochromator

Technical specifications

The main components of the device are: laser-plasma source (LPS) of soft X-ray and extreme ultraviolet radiation; Czerny-Turner spectrometer/monochromator (CSM) with a flat diffraction grating (FG); probing beam formation system (PBS); goniometer (G); vacuum pumping and automation of control processes.

Scheme of a reflectometer with a laser-plasma source and a Czerny-Turner monochromator

The operation of the device is as follows. Radiation from a Nd:YAG laser (Expla NL-300, λ = 1.06 μm, pulse energy 0.5 J, duration 4 ns, pulse repetition rate 10 Hz) operating in the Q-switched mode using a short-focus lens L (focal length 45 mm) is focused on the target into a spot with a diameter of about 100 μm (power density ~ 1012 W/cm2). The average energy of the laser pulse is controlled using a thermoelectric calorimeter (K), which receives part of the radiation reflected from the separating plate (RP). The prism (P) redirects the laser beam into the target chamber. Radiation is introduced into the radiation source chamber through the optical input (OI) into a vacuum. The quartz plate (QP) protects the lens from contamination by target erosion products.

The radiation emitted by the plasma torch falls on the entrance slit Щ1 of the spectrometer/monochromator. The monochromatic probing beam enters the SPZP chamber for forming and monitoring the probing beam intensity. The probing beam intensity is controlled using a detector-monitor (Mon) installed between the exit slit (Щ2) and the toroidal mirror (TZ). The toroidal mirror forms a probing beam of a given size on the sample under study. The sagittal and meridional radii of the TZ are selected so that the TZ forms an image of the radiation source in the vertical plane and of the exit slit Щ2 in the horizontal plane. Since the position of the slits and the radiation source does not change during spectral scanning, the geometric characteristics of the probing beam on the sample do not depend on the wavelength. The angle of incidence of the radiation on the TZ is fixed and is 1.5°. Gold was chosen as the TZ coating with an average reflectivity of 80% over the entire working range.

A separate thin-film absorption filter (F) is used to suppress high harmonics (at least the very effective second harmonic) of the diffraction grating.

The probing beam, reflected from the TZ and passed through F, is focused into the center of the goniometer, on which the studied sample is installed. Since most samples have a curved shape of the reflecting surface, and the numerical aperture can reach NA = 0.5, the goniometer has 7 degrees of freedom (five for the sample and two for the detector, providing positioning of any point along the goniometer axis and orientation along the beam axis. The reflectometer provides all the necessary types of research: spectral dependences of reflection and transmission coefficients at a fixed position of the sample; measurement of angular dependences of reflection coefficients at any point of the sample.

Operating principle of the spectrometer-monochromator

When developing the X-ray spectrometer-monochromator, the Czerny-Turner scheme with two collimating mirrors and a flat diffraction grating was chosen. The operating principle of the spectrometer-monochromator is as follows. The input slit Щ1 and the output Щ2 slits are located at the meridional foci of mirrors З1 and З2, respectively. The diverging radiation beam leaving the slit is transformed by mirror З1 into a parallel beam incident at an angle  (hereinafter, grazing angles) on the flat diffraction grating. The diffracted parallel monochromatic beams fall on mirror З2 and are focused in its focal plane. The beam diffracted at an angle β is focused on the output slit Щ2. The angles of incidence of the beams on the mirrors in our case are equal to γ1 = γ2. In this case, the axes of the incident and emerging beams from the monochromator coincide, which makes it possible to change the operating range of wavelengths without changing the relative position of the reflectometer elements.

Scanning the spectrum is accomplished by rotating the grating, in accordance with the diffraction grating equation:

where m is the diffraction order, λ is the wavelength and D is the grating period, α and β are the angles of incidence and diffraction, respectively.

For the practical implementation of such rotation, a system of a hinge and a lever is used. The formulas for the correspondence of the linear movement of the lever pusher and the actual angle of rotation of the diffraction grating are also given there.

Hinge and lever system for rotating the diffraction grating