W / B4C multilayer (ML) mirrors with varying periodicities ( d ) = 1.6 to 5.4 nm are tested for rapid thermal and temporal stability, which are required for space-based x-ray telescopes for astronomy. The aging effects on the structural parameters over a period of 2 years are assessed through hard x-ray reflectivity (HXR) measurements. Multiwavelength performance of ML mirrors is studied over thermal cycling from −40 ° C to +50 ° C for 1, 3, and 10 days, which simulate the expected temperature variation in the low-earth orbit. The structural parameters of all samples remained nearly constant over the first 2 years. It is observed that the short-period MLs develop a contamination layer over time. Rapid thermal cycling results indicate no change in HXR for all ML mirrors. However, at soft x-rays, there is a reduction in reflectivity after thermal cycling. The variations in optical performance at hard and soft x-ray energies after thermal cycling are due to variation in interface roughness at different spatial frequencies.
We present fabrication and structural analysis of two different multilayer grating structures. W/B<sub>4</sub>C based lamellar multilayer grating (LMG) was studied for high resolution monochomator application near soft x-ray region (~1.5 keV). Whereas NbC/Si based multilayer phase-shift reflector (MPR) was studied for high reflection at normal incidence near Si L-edge (~99 eV) and simultaneously to suppress the unwanted vacuum ultraviolet / infrared radiation. The grating patterns of different periods down to D = 10 micron were fabricated on Si substrates by using photolithography, and multilayers (MLs) of different periodicity (d = 10 to 2 nm) and number of layer pairs (15 to 100) were coated using sputtering techniques by optimizing the process parameters. The LMG and MPR samples are characterized by x-ray reflectivity (XRR) and atomic force microscopy (AFM) measurements. XRR results show successive higher order Bragg peaks that reveal a well-defined vertical periodic structure in LMG, MPR and ML structures. The lateral periodicity of the grating and depth of the rectangular groves were analyzed using AFM. The AFM results show good quality of lateral periodic structures in terms of groove profile. The effect of the process parameters on the microstructure (both on vertical and lateral patterns) of ML, LMG and MPR were analyzed.