A new coherent soft X-ray branchline at the Advanced Light Source facility has been designed and is near completion. Using the third harmonic from the U8 undulator, this branch will operate from 200 eV to 1000 eV. This increased energy range will allow us to bring interferometric techniques previously used in the extreme ultraviolet region to the soft X-ray region.
Coherent radiation from undulator beamlines has been used to directly measure the index of refraction of several metals [1,2]. We have used this same interferometric technique to measure the indices of refraction of silicon, ruthenium, and TaSiN, materials important for EUV lithography. We now wish to use this technique to directly measure the refractive index of materials with absorption edges in the 500-800 eV range.
Also, we have begun work to use an at-wavelength phase shifting point diffraction interferometer (PS/PDI) to measure the quality of zone plates at soft X-ray wavelengths (below the oxygen K edge of 543.1 eV). The results of this experiment will aid in the evaluation of zone plates currently used in soft X-ray microscopy
The soft X-ray microscope XM-1  at Lawrence Berkeley National Laboratory (LBNL) is a full-field transmission microscope (Figure 1). It has the unique capability to image wet samples as thick as 10 µm in air, while it provides elemental and chemical contrast over a 10-µm-in-diameter object field. All of these features are accompanied by very high resolution (20 nm), which is made possible by the fine features of the micro zone plate (MZP) . XM-1 has been found to be very useful in biological and material studies. Recently, material scientists have taken advantage of XM-1's special capabilities to investigate the magnetic properties of different materials (e.g., Co and Fe).
My research is to measure and improve the resolution of XM-1, which depends in part on the performances of the MZP. Several test objects containing line and space patterns (grating) with different periods and duty cycles were utilized to measure the contrast as a function of spatial frequencies. Recently, we employed multilayers, a structure of two interleaving material layers, for resolution measurement because of their precisely controlled structure quality. Figure 2 shows the X-ray image of a 25 nm Si/25 nm Cr multilayer imaged at 600 eV. These measurements were compared with the simulation results obtained from a program called SPLAT, which was developed by Professor Andrew Neureuther's group in our department. One way to enhance the resolution of the microscope is to reduce the outermost zone width and increase the absorption of light by the opaque zones using thicker zones, which demand a high aspect ratio. A bilayer resist process in which the zone pattern was written into a high-resolution resist layer and then transferred to a thick hardbaked polymer by ICP plasma etching, has successfully yielded 5:1-aspect-ratio MZPs with the smallest zone width of 35 nm . Currently, an effort to decrease the zone width is being undertaken.
Figure 1: The layout of the soft X-ray microscope XM-1 at beamline 6.1.2 of LBNL's Advanced Light Source (ALS) synchrotron radiation facility. Bending magnet radiation from the ALS is used to illuminate the sample.
Figure 2: The soft X-ray image of 25 nm Cr/25 nm Si multilayer cross-section at 600 eV.