**6. Sample fabrication**

In the conventional approach, X-ray holography samples are integrated with the holographic masks, leading to the unique advantage of inherently drift-free imaging at the price of exceptionally complex sample fabrication. There is a concept to separate mask and specimen [2, 3], but it requires giving up drift-free imaging and hence the main argument for using X-ray holography instead of, e.g., STXM. Therefore, in the foreseeable future, the fabrication of advanced integrated samples will remain one of the main challenges of X-ray holography from an end users perspective. A detailed recipe has been published in [10], which will be briefly reviewed here.

Despite some demonstration of X-ray holography in reflection geometry [8], most present day applications of the technique rely on transmission measurements of transparent samples. Soft X-rays have a sub-micrometer penetration depth in any solid material. Hence, free standing films, the so-called membranes, are required to support the specimen, which itself needs to be thin. The most wide-spread approach is to use commercially available silicon nitride (SiN) membranes. The thickness of the membrane needs to be adapted to the specific imaging conditions, in particular to the photon energy. For instance, for magnetic imaging at the transition metal M edges (around 60 eV), anything more than a few tens of nanometers would already be opaque. For measurements at energies exceeding 500 eV, thicknesses of 500 nm and more are feasible and often even advisable because of the increased stability and robustness against various fabrication hardships. For the same reason, membranes should not be unnecessarily large, that is, their size should be just enough to fit the object and reference holes.

**Figure 4** illustrates the steps to fabricate a sophisticated sample that is suitable for timeresolved imaging of field-induced magnetic domain dynamics [10, 20]. Not all of these steps are required for every sample. However, all holographic samples start with a membrane (step a) with an opaque layer on the back side (step b). [Cr(5)/Au(55)]20 has proven suitable for imaging at 778 eV, where the transmission of this stack is below 10−<sup>9</sup> . The absorption is mainly due to the gold. The chrome layers prevent the formation of large Au grains, which significantly eases focused ion beam (FIB) milling of the material. Steps c, g, and h, that is, FIB milling of the object and reference holes, are also required in any holography sample process. In principle, any process can be used to prepare a specimen on the front side of the membrane as long as ultrasonic exposure is avoided. In **Figure 4**, the remaining steps are marker fabrication (step d), material deposition and shaping (step e), and patterning of a microcoil (step f).

**6. Sample fabrication**

232 Holographic Materials and Optical Systems

**Figure 3.** Top-view schematic of a holographic imaging chamber.

In the conventional approach, X-ray holography samples are integrated with the holographic masks, leading to the unique advantage of inherently drift-free imaging at the price of exceptionally complex sample fabrication. There is a concept to separate mask and specimen [2, 3], but it requires giving up drift-free imaging and hence the main argument for using X-ray holography instead of, e.g., STXM. Therefore, in the foreseeable future, the fabrication of advanced integrated samples will remain one of the main challenges of X-ray holography from an end users perspective. A detailed recipe has been published in [10], which will be briefly reviewed here.

Despite some demonstration of X-ray holography in reflection geometry [8], most present day applications of the technique rely on transmission measurements of transparent samples. Soft X-rays have a sub-micrometer penetration depth in any solid material. Hence, free standing films, the so-called membranes, are required to support the specimen, which itself needs to be thin. The most wide-spread approach is to use commercially available silicon nitride (SiN) membranes. The thickness of the membrane needs to be adapted to the specific imaging conditions, in particular to the photon energy. For instance, for magnetic imaging at the transition metal M edges (around 60 eV), anything more than a few tens of nanometers would already be opaque. For measurements at energies exceeding 500 eV, thicknesses of 500 nm and more are feasible and often even advisable because of the increased stability and robustness against various fabrication hardships. For the same reason, membranes should not be unnecessarily

large, that is, their size should be just enough to fit the object and reference holes.

imaging at 778 eV, where the transmission of this stack is below 10−<sup>9</sup>

**Figure 4** illustrates the steps to fabricate a sophisticated sample that is suitable for timeresolved imaging of field-induced magnetic domain dynamics [10, 20]. Not all of these steps are required for every sample. However, all holographic samples start with a membrane (step a) with an opaque layer on the back side (step b). [Cr(5)/Au(55)]20 has proven suitable for

. The absorption is mainly

**Figure 4.** Steps for fabrication of a holographic sample. (a) Starting with a commercially available SiN membrane of a few micro meters in lateral dimensions. (b) An opaque Cr/Au multilayer is deposited on the topographic side of the membrane. (c) The Cr/Au is removed in a circular region, defining the object hole. The SiN is not removed. (d) Alignment markers are fabricated to align subsequent steps with respect to the object hole. (e) A disk-shaped magnetic specimen is fabricated behind the object hole. (f) A microcoil is wrapped tightly around the magnetic specimen to generate short magnetic field pulses. (g) The SiN is removed in large squarish areas where the references are supposed to be located. The removal of SiN makes the fabrication of small references significantly easier. (h) References are prepared in the previously designated areas. Reproduced from Ref. [10].

The result of this process can be seen in **Figure 5**. Note that optical lithography or even simple shadow-masking can be used to considerably simplify the overall process if sub-micrometer alignment precision is not required.

**Figure 5.** Scanning electron micrographs of a holographic sample. (a) Cross section of the PMMA resist used to fabricate the microcoil. Steep walls are required to prepare the coil in close vicinity to the magnetic specimen. (b) View of the final sample under 52°. The disk-shaped element in the center is the magnetic specimen. The black halo around it is the object hole visible through the SiN membrane. Wrapped around the sample is a gold microcoil. Three large square pads are milled into the SiN membrane to facilitate the fabrication of reference holes. One of the reference holes was milled from this side of the sample, here visible as a large conical aperture. (c) Top view of the same sample, now also showing the second reference in the bottom-left pad. This reference was milled from the other side of the sample and the exit is much more constricted then the entrance. The properties of the reference wave depend significant on whether the beam first passes the conical part or first the constricted part [14]. Reproduced from [10].
