Beam splitters. Another multilayer optical component of great interest is the soft x-ray beam splitter. At visible wavelengths beam splitters play a vital role in holographic, interferometric, and schlieren systems. They are important as output couplers for laser cavities, are useful for beam monitoring functions, and in general provide versatility in sophisticated optical design.
At Livermore Laboratory we have pioneered the development of multilayer beam splitters for use at soft x-ray wavelengths (39, 40, 53, 56, 57). Using silicon nitride membrane technology (58), originally developed by the integrated circuit industry for x-ray lithography, we have designed, fabricated, and tested Mo/Si multilayers supported by thin (about 300 angstroms), x-ray transparent, silicon nitride membranes. Details of beam splitter fabrication are provided elsewhere (40, 56). In brief, a polished silicon wafer is coated (via chemical vapor deposition) with a thin silicon nitride film onto which a Mo/Si multilayer is deposited (via magnetron sputtering). Using conventional patterning and anisotropic etch techniques (59), the underlying silicon is removed from an area typically 10-20 mm squared, leaving a window of silicon nitride supported multilayer, which is partially replecting and transmitting as illustrated in Fig. 8.
Currently, the major challenge in producing high-optical-quality soft x-ray beam splitters lies in achieving a smooth interface between the silicon nitride (or other transparent support membrane) and the multilayer structure. It is not uncommon for the CVD silicon nitride film to have a surface roughness of order 10 to 15 angstroms. The roughness is replicated in the multilayer structure resulting in a reduced reflectivity (60), and possible wavefront distortion of the reflected beam. While efforts are under way to produce smoother support films, improvements in beam splitter performance have already been achieved by depositing the multilayer onto the back side of the silicon nitride membrane, after the silicon has first been anisotropically etched. In this procedure the silicon nitride film is deposited onto the highly polished silicon wafer. The interface between the silicon nitride and the silicon appears to be almost as smooth as the polished wafer. After the silicon has been etched away, the Mo/Si multilayer is then deposited onto the "smooth" surface of the silicon nitride that had been in contact with the polished silicon. Figures 9a and 9b compare TEM micrographs of Mo/Si beam splitters deposited on the "rough" top side (Fig. 9a) and on the "smooth" bottom side (Fig. 9b.) of the silicon nitride. In Fig. 9a the beam splitter specifications are Mo/Si, d=115 A (angstroms), y is about 0.51, N=7, silicon nitride thickness about 440 A. In Fig. 9b the beam splitter specifications are Mo/Si, d=65 A, y is about 0.4, N=13, silicon nitride thickness is about 3000 A. Figure 9 shows the reduced roughness in the "bottom-side" deposited multilayers. Figure 10 illustrates the comparative performance of "top-side" and bottom-side deposited multilayers versus a multilayer deposited onto a bare, highly polished silicon wafer. All samples were prepared in the same run of the magnetron sputter system. Nominally they are all Mo/Si multilayers with d=65 A, y is about 0.3, N=13. As shown, peak reflectivities are 17, 13.4, and 9.7% for the multilayers deposited onto the polished wafer, the bottom, and the top sides of the silicon nitride, respectively. These results indicate that the bottom-side deposited beam splitters, while better than the top-side deposited optics, are not yet as good as has been achieved with multilayers deposited onto highly polished substrates. (Note also the slight shift in the d spacing for the bottom-side deposited multilayer. This is apparently due to shadowing of the substrate surface by the sidewalls of the etched silicon.)
The soft x-ray beam splitters are new optical components, still in the earlyt stages of development. First generation devices were developed for x-ray laser applications at about 200 and about 130 A and have been performance limited by the poor optical quality of the silicon nitride support membranes. Current research is focused on extending beam splitter operation to shorter wavelengths and finding new xray transparent support membranes which are both strong and optically smooth. In both these efforts newly developed technologies from the microfabrication industry are playing an important role.