2.2.1.

Gas-discharge light source

In our system, we adopted an RF-induced gas-discharge lamp (FERMION INSTRUMENTS) based on local field plasma mechanism as the light source, which is electrode-free and ignite-free. A schematic of the lamp, as shown in Fig. 2, shows four parts in general: (1) a solid RF source with the central working frequency of 433 MHz and the maximum power output of 200 W. The optimal working frequency should couple with the position of focused electric field and should be fine adjusted according to the gas type and working pressure. (2) The capillary design is optimized to the resonant cavity which consists of a bulb and a tube. The bulb is used as the main gas discharge area, and the tube is used as an entrance for the gas inlet and access for the light transmission. (3) Gas supply system which ensures pure gases and controllable gas pressure. (4) Cooling system including water cooling and air cooling.

Fig. 2

Schematic of the gas-discharge light source.

The lamp has a small weight (only 3 kg) and it can accommodate helium, neon, argon, krypton, xenon, nitrogen, oxygen, etc., or a mix of the gases. By measuring the intensity of the exit EUV light beam from the tube, it was estimated that more than 5E+12 photons could be generated per second for Ne II spectral line (46.07 nm and around) when the RF power of 100 W and neon gas pressure of 2E–4 mbar was used. The divergence of the exit light beam is only about 2 deg which is very small and makes it compatible with monochromators. Using the spectral lines or relatively strong continuous spectra of the gases, the reflectometer is able to operate in the wavelength bands from 30 to 200 nm. Such a broad working band is exciting because the probe beam in FUV band is relatively difficult to be acquired in laboratory-based reflectometers.³⁶ The effect of higher diffraction order can be greatly reduced when only the discrete characteristic lines of the working gases are used for tests.

Figure 3 shows discharging spectra from 30 to 200 nm of several gases including neon, argon, krypton, carbon dioxide, nitrogen, and oxygen. Some strong characteristic lines are marked with the electronic energy level transitions and the corresponding wavelength available in the NIST website. All spectra were measured with the entrance and exit slits of $300\mu \mathrm{m}$.

Fig. 3

Spectra of several atomic and melecular gases with $300\text{-}\mu \mathrm{m}$ entance and exit slit of the monochromator.

2.2.2.

Toroidal grating monochromator

Horiba TGM1200 was adopted as the monochromator with a 1-m focal length and 146 deg deviation angle between the entrance and exit arms. In the chamber, there are two switchable varied-line-spacing gratings working for different wavelength range, one is a 250-gr/mm grating coated with Pt used for 50- to 200-nm wavelength range, while another Au coated grating with line density of 950 gr/mm is of high efficiency in 15- to 50-nm wavelength range.³⁷ Both gratings are concave toroidal with the slope errors controlled at about 0.2 arc sec both in the directions parallel and perpendicular to the grooves, to ensure excellent performance for correcting aberrations and reducing the quantity of focusing mirrors in the optic path, thus the resolution and throughput of the monochromator can be improved.

In Fig. 4, we studied how the spectral resolution and exit light intensity vary with the slit width of the monochromator. With different configured entrance and exit slit widths, the complete intensity peaks of the characteristic lines are measured by a Si photodiode detector positioned right behind the exit slit. To maintain the spectral scanning with a step of 0.06 nm, the grating rotation and data collection are controlled by a customized LabVIEW program. For each characteristic line, the RF power and gas pressure are required to keep stable during measurement. The spectral resolution as presented by FWHM is deduced by fitting the peaks with a Gauss profile, and the monochromatic light intensities are determined by the peak values. The spectral resolution reaches as high as 0.27 nm at 46.07 nm with $10\text{-}\mu \mathrm{m}$ entrance slit and $30\text{-}\mu \mathrm{m}$ exit slit, and 0.28 nm at 123.58 nm with $50\text{-}\mu \mathrm{m}$ entrance slit and $30\text{-}\mu \mathrm{m}$ exit slit, but the intensity of the exit beam was too weak to be used for reflectivity measurements presently using the photodiode detector. Considering the fact that we used characteristic lines rather than continuous spectra, the width of the slits does not affect the spectral resolution so much. So we adjusted the entrance and exit slit at $300\mu \mathrm{m}$ where the spectral resolution moderately decreases to 0.44 nm at 46.07 nm and 0.46 nm at 123.58 nm with the intensity increased by more than 100 times and 20 times, respectively, compared with the situation of the highest resolution. With the entrance and exit slit of $300\mu \mathrm{m}$, the throughput of the monochromator is about $4.6\times {10}^{10}$ photons per second (PPS) at 46.07 nm and $8.2\times {10}^9$ PPS at 123.58 nm.

Fig. 4

Spectral resolution and exit beam intensity variation as a function of exit slit width at 46.07 nm with entrance slit width of (a) 300, (b) 100, (c) 30, and at 123.58 nm with entrance slit width of (d) 300, (e) 100, and (f) $50\mu \mathrm{m}$.

2.2.3.

Collimation chamber

A collimation chamber is coupled to the monochromator exit flange. Inside this chamber, an Au coated customizing designed toroidal mirror, with 11.3 arc sec slope error in the long side direction and 53.1 arc sec in the short side direction, was mounted on a six-dimensional (6D) adjustment platform. The beam exit from the monochromator is directed onto the mirror with incident angle varied from 69.34 deg to 70.66 deg. Within this incident angle range, the Au film keeps relatively high reflectivity in 30- to 200-nm wavelength region, as shown in Fig. 5. The 6D adjustment platform enables the mirror to be precisely transferred along $X$ and $Y$ axis ($X$ axis is perpendicular to mirror surface and $Y$ axis is parallel to mirror surface), transferred vertically along $Z$ axis, rotated with respect to $Z$ axis, tipped with respect to the $X$ axis and tilted with respect to the $Y$ axis. The stability of the platform has been proved by observing the position of the beam using a UV CCD mounted on the sample chamber far away and no obvious shift has been found for several months.

Fig. 5

S-polarized (red lines), p-polarized (blue lines) and average (black lines) reflectivity of Au coating in 30- to 200-nm wavelength range at minimum incident angle of 69.34 deg (dotted lines), central incident angle of 70 deg (solid lines) and maximum incident angle of 70.66 deg (dash lines).

The incident beam is reflected by the collimator mirror with an approximate area of $8\mathrm{mm}\times 8\mathrm{mm}$ perpendicular to the optical path, which is further collimated by two sets of collimation slits sited at the positions of about 230 and 1260 mm away from the mirror. The collimator mirror and two slits reduced the divergent angle of beam from 1.32 deg to $-0.01$ deg in meridian direction (in the $XOY$ plane) and from 1.23 deg to 0.08 deg in sagittal direction (in the $XOZ$ plane), according to experimental measurements. As the angular scans of the samples are performed in $XOY$ plane, the divergence in the sagittal direction does not affect the angular resolution in the experiments.

2.2.4.

Modulation/monitor chamber

To measure extremely weak reflected beam with the help of lock-in amplifiers, it is necessary to modulate the temporal quasi-continuous light into periodic. For creating periodically modulated light as well as monitoring the light intensity, a two-sector optical chopper disk was mounted in the chamber with Au-coated polished glass cemented on the blades. On the one hand, the periodic signal detected by a groove photoelectric sensor mounted under the blades serves as a reference input for lock-in amplifiers, and the lock-in amplifiers then single out the component of the signal at the reference frequency using a phase-sensitive detection technique so that very small AC signals down to a few pico-amperes can be measured. In our system, the maximum modulating frequency is 60 Hz with the disk rotating 30 revolutions per second. On the other hand, the chopper functions as a beam splitter. The disk chops the light beam at an incident angle of 70 deg, and the reflected reference beam was detected by a Si photodiode (AXUV100G type) to monitor the intensity of the incident beam.

2.2.5.

Sample chamber

The sample chamber is a 1.5-m high and 1.2-m diameter chamber housing a high-precision translation stage consisting of a three-dimensional linear stage, an angular stage, and a goniometer assembly with six degrees of freedom which were marked in Fig. 1, including $\mathrm{X}$ (vertical to the optical path direction in horizontal surface), $Y$ (along the optical path direction), $Z$ (vertical to horizontal surface), $\phi$ (sample inclination), ${\theta }_s$ (sample rotation) and ${\theta }_d$ (detector rotation), respectively. The ${\theta }_s$ and ${\theta }_d$ have concentric rotation axis perpendicular to the XOY plane with resolution within 4″ from 0 deg to 360 deg range. The sample is cemented on a bracket that should be inserted into a fixed slot structure. The sample surfaces were initially parallel to the YOZ plane which corresponds to ${\theta }_s=0\mathrm{deg}$. Another twin Si photodiode (AXUV100G type) is used as the detector. The detector was mounted on a rotation arm assembly which was about 430 mm away from the rotation axis and was initially placed at the end of the light beam right behind the sample, which was set as ${\theta }_d=0\mathrm{deg}$, as shown in Fig. 1. The detector can rotate dependently or independently with the sample. $X,Y$, and $Z$ motions have a resolution of 3 μm over its range of ±50 mm. The tilt motion controls the sample inclination relative to Y axis, and its resolution is better than 3″ within the angular range of $\pm 20\mathrm{deg}$. Currently, the samples are cemented on a bracket alone $Z$ direction, thus the $Z$ motion can be used to switch the samples. $X$ and $Y$ motions would be applied if the samples need to be translated out of the beam. The stage allows a maximum sample diameter of 100 mm and a weight capacity of 30 kg.

The sample chamber connects with the former chambers by a manual valve. When closing the manual valve, the sample chamber can be vented alone, which is quite convenient for fast changing samples.

4.2.1.

Angular dependence of reflectivity

Sc/Si multilayer has excellent reflectivity at the wavelength of 35 to 50 nm and is expected to be applied in the solar EUV telescope for reflecting Ne VII line (46.5 nm) at normal incidence angle.³⁸ The tested sample was designed as [Si (6.77 nm)/Sc (15.65 nm)]₂₀/Si-sub with a top silicon oxide layer estimated to be around 2 nm,³⁹ to work at 46.5 nm with the normal incidence angle of 4.6 deg. The sample was deposited on a $20\mathrm{mm}\times 20\mathrm{mm}$ polished Si wafer by DC magnetron sputtering and stored with 15% humidity.

Its angular dependence of the reflectivity was measured and shown in Fig. 10 from 3 deg to 20 deg incident angle with a step of 1 deg. At each angle, both the reflected light intensity and reference light intensity was sampling with 512 Hz and subtracted by background signal. The reflected light intensity was normalized to the reference signal. Theoretical reflectivity calculated by IMD software⁴⁰ was also given in Fig. 10 which has preliminarily verified the experimental results were reasonably close to the expected values.

Fig. 10

Reflectivity curve with changing incident angle from 3 deg to 20 deg.

4.2.2.

Spectral dependence of reflectivity

$\mathrm{Al}/\mathrm{LiF}/{\mathrm{MgF}}_2$ film was distinguished by its excellent reflective performance in FUV band thus worth researching for further applications in many astrophysical missions. We have tested a piece of $\mathrm{Al}/\mathrm{LiF}/{\mathrm{MgF}}_2$ film sample utilizing relatively strong characteristic line spectrum of several atomic and molecular gases. For calibration, we also measured its reflectivity at BESSY-II synchrotron and HLS, which is given in Fig. 11. Compared with average test value of BESSY-II synchrotron, the deviation is $<9\%$, while compared with tests of HLS, the deviation is $<5\%$. The individually shift deviations may be caused by some very weak spectral lines, consequently, the reflected light intensity would be affected slightly by the fluctuation of the background intensity. At wavelength around 105 nm, results of HLS are relatively unreliable because of the influence of higher diffraction orders.

Fig. 11

Reflectivity value of $\mathrm{Al}/\mathrm{LiF}/{\mathrm{MgF}}_2$ film at different wavelength and compared with tests from BESSY-II and HLS.

4.2.3.

Stability and repeatability

For verifying system stability, we have performed the same experiment for nine times and acquired a 0.5% RMS value. In addition, with all the experimental conditions unchanged, we carried out the same experiments for days’ interval to test system repeatability. Results have demonstrated that the repeatability is better than 2%.