B₄C/Pd multilayers with small d-spacing can easily degrade in the air, and the exact degradation process is not clear. In this work, we studied the chemical modification of B₄C films and B₄C/Pd double layers stored in four different environments: a dry nitrogen environment, the atmosphere, a dry oxygen-rich environment, and a wet nitrogen environment. The XANES spectra of the B₄C/Pd layers placed in a dry oxygen-rich environment showed the most significant decrease in the σ* states of the B–C bonds and an increase in the π* states of the B–O bonds compared with the other samples. X-ray photoelectron spectroscopy (XPS) measurements of the samples placed in a dry oxygen-rich environment showed more intensive B-O binding signals in the B₄C/Pd layers than in the single B₄C film. The results of the Fourier-transform infrared spectroscopy (FTIR) showed a similar decrease in the B–C bonds and an increase in the B–O bonds in the B₄C/Pd layers in contrast to the single B₄C film placed in a dry oxygen-rich environment. We concluded that the combination of palladium catalysis and the high content of oxygen in the environment promoted the oxidization of boron, deteriorated the B₄C composition.

With the rapid development of optical systems, aspheric reflective optics have become more and more widely used because of their advantages in obtaining better imaging quality. Meanwhile, the optical systems have higher requirements in terms of the surface precision of their optical elements. In this study, we proposed an improved profile-coating method to realize a two-dimensional surface correction method on a rotational symmetric hyperboloid mirror. This method used an irregular mask based on a planetary motion magnetron sputtering system to control film thickness distribution. Moreover, film thickness calibration with a step test was carried out to reduce the processing error of the mask. An optical profiler was used in the step test to quantitatively characterize film thickness distribution and a tilt correction was introduced to correct the test error. As a result, an improvement in figure error in the radial direction of 17.7 nm Root Mean Square (RMS) was achieved. According to these optimization methods, the mask was trimmed for film deposition on the spherical surface. Measurement results from the interferometer show that the figure error of film was 16.23 nm RMS, demonstrating the effectiveness of the optimized method for fabricating a rotational symmetric hyperboloid mirror.

Pd/B₄C multilayers with 2.5 nm of d-spacing and thick Si and B₄C capping layers were fabricated to study temporal stability under storage in different environments with relative humidity of 10% and 50%. The two stored samples were investigated using grazing incidence X-ray reflectometry (GIXR), X-ray scattering (XRS), an optical microscope, transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The GIXR results showed that the reflectivity of the samples under 10% humidity and 50% humidity dropped by 3% and 8%, respectively, after 13 months. The optical microscope showed that the surface of the 10% humidity sample was smooth and undamaged, whereas the surface of the 50% humidity sample significantly eroded. TEM showed that the internal multilayer structure of the sample stored in 10% humidity was well protected by the capping layers. For the sample stored in 50% humidity, a major part of the Si and B₄C capping layers were wrinkled and delaminated, and some surface layers of the multilayer structure were degraded with severe diffusion of boron. The XPS results showed a relatively large amount of oxygen in the B₄C capping layer of the 50% humidity sample, and an obvious oxidation of the boron was found in the B₄C capping layer and the surface of the multilayer. The severe oxidation and diffusion of boron and the delamination of the capping layers caused the degradation of the Pd/B₄C multilayers stored in 50% humidity.